I suspect there are no written rules about it.
A quick and dirty heuristic could be one of the following:
Skill ranks are training of the body and mind. You retain any skill point spent on mental skills (Int- Wis- and Cha- based ones). You use host's skill points on physical skills: even the Dex- based Thievery is stroed in a sort of body memory so that your fingers actually move by themselves when you are under pressure.
Skill ranks are applied knowledge. You retain all your skill points and ability modifiers on mental skills. You apply half (rounded down) of your skill points and the full host's ability modifiers on physical skills.
On the feat side, we could trace a rough line by stating that pure mental feats are those that don't enhance or modify any of the following stats:
- Hit points
- AC
- Damage reduction
- Damage dealt (if bludgeoning, slashing or piercing)
- Save DC of natural or extraordinary abilities driven through a melee or ranged attack (such as those of the Stunning Fist)
- Fortitude and Reflex saving throws
- Spell Resistance
- Forms of movement
- Physical skills
- Other statistics related to physical abilities (such as the maximum encumbrance)
Furthermore, any host's feat that provides additional maneuvers during melee or ranged attacks, or that provides or enhances other possibilities for changing position or location will be shut down. Possible examples:
- Step Up
- Rapid Shot
- Stunning Fist
- Power Attack
Many of the feats, as you already noticed, are very borderline. However, I'd prefer to be restrictive rather than permissive.
As written, the witch archetype beast-bonded's supernatural ability twin souls kills a possessed foe
The spell magic jar has a series of clauses making missteps extremely dangerous, both for the possessor and the possessed, but the beast-bonded witch's supernatural ability twin souls changes that effect with three parenthetical words. It says that
...if the witch or her familiar is gravely injured or about to die, the soul of the dying one immediately transfers to the other’s body. The two souls share the surviving body peaceably, can communicate freely, and both retain their ability to think and reason. The host may allow the guest soul to take over the body temporarily or reclaim it as a move action. They can persist in this state indefinitely, or the guest can return to its own body (if available) by touch, transfer into a suitable vessel (such as a clone), or take over another body as if using magic jar (with no receptacle).
Emphasis mine. First, for the ability to activate, the GM must determine that either the witch or the familiar is either gravely injured or about to die (which means, I assume, the creature has the condition dying but ask the GM).1
Assuming the witch activates the special ability twin souls by whittling himself to dying 1 point of damage at a time (the witch doing this to himself rather than to his familiar because the witch needs the familiar to prepare spells), the witch then inhabits the familiar's body as per the description of the ability twin souls and can use an effect like the spell magic jar "with no receptacle."2
What this parenthetical means is deeply unclear. This 2012 thread offers one interpretation. Below I present an alternative that combines the special ability twin souls and the spell magic jar stripped of all references to the receptacle. Such an ability reads...
You can attempt to take control of a nearby body, forcing its soul out of its body, killing it if it has nowhere to go. The spell ends when you send your soul back to your own body.
Attempting to possess a body is a full-round action. It is blocked by protection from evil or a similar ward. You possess the body and force the creature's soul out unless the subject succeeds on a Will save. Failure to take over the host leaves your life force in the current body, and the target automatically succeeds on further saving throws if you attempt to possess its body again.
If you are successful, your life force occupies the host body, and the host's life force is pushed out. You keep your Intelligence, Wisdom, Charisma, level, class, base attack bonus, base save bonuses, alignment, and mental abilities. The body retains its Strength, Dexterity, Constitution, hit points, natural abilities, and automatic abilities. A body with extra limbs does not allow you to make more attacks (or more advantageous two-weapon attacks) than normal. You can't choose to activate the body's extraordinary or supernatural abilities. The creature's spells and spell-like abilities do not stay with the body.
If the host body is slain, you die, and the life force of the host departs (it is dead). Any life force with nowhere to go is treated as slain.
If the spell ends while you are in a host, you return to your body (or die, if it is out of range of your current position).
Alternatively, you can take a move action to touch your body to return your life force to it.
...And while that seems crazy, it's not quite as crazy as it sounds. Yes, this magic jar effect is a save-or-die, but a beast-bound witch must be at least level 10 to use this ability, which means save-or-die effects have been available for at least 3 levels (e.g. phantasmal killer). Further, the witch had to render himself dying first to have access to the ability. Also, once the magic jar effect is used successfully, the only way the witch can use the ability again is by rendering the new body dying again, transferring to the familiar, and finding another body to possess; the ability doesn't appear to work while the witch is possessing a creature, but ask the DM. Finally, the duration of the magic jar effect remains: it's 1 hour/level, and the only longer-lasting switch the witch can make is to his own body or a "suitable vessel," a term the GM must also clearly define.
(Remember, too, that once the witch fails to possess an individual, the witch can never possess that individual. That might be a hassle.)
Officially, however, what the ability should do is mysterious and in the GM's hands. Talk with the GM first before attempting to exploit it. Also, I urge you to press the FAQ button on this post—as I did—to encourage Paizo's development team to address the beast-bound witch's twin souls ability in a future FAQ.
1 This GM would rule that an effect that straight-up, flat-out kills the witch or familiar still leaves the witch ding-dong dead, and the twin souls ability goes unused. Being gravely injured or about to die is a thing, but dead is dead. This GM doesn't view the the special ability twin souls as having a kind of limited omniscience, but another GM may. Such is a risky tack to take, however, possibly making a nigh-unkillable character, one that keeps his familiar in a secure yet comfortable lair miles (or continents or planes) away from any direct engagement, and who uses acquired bodies to depart the lair and harry his foe. This is, essentially, a dirt cheap (and likely murderous) variant of astral projection yet 7 levels early.
2 My gut says that with no receptacle should've said using the witch's or the familiar's body as a receptacle yet space constraints forbade that language. But that is totally speculation on my part and isn't what the ability says.
Best Answer
This spell has nothing to do with magic jar, except that the effect is somewhat similar. This spell does what its text says and nothing more or less than that. Based on your questions, it sounds like you might be confused as to how spells work in general. When you cast a spell with a target, you choose a valid target in range to whom you have line of effect and that target is the target of your spell. If you choose an invalid target, the spell doesn't work. Spells can overwrite these mechanics, but they have to specify that they do so. Marionette possession does not specify any changes to the normal way spells are cast, so there aren't any changes from that.
Specific responses:
It's by line of effect, but that's not how line of effect works. "Line of Effect: A line of effect is a straight, unblocked path that indicates what a spell can affect. A line of effect is canceled by a solid barrier. It's like line of sight for ranged weapons, except that it's not blocked by fog, darkness, and other factors that limit normal sight." source
No, that's not why. An unconscious creature would still not get a save, because they are willing. The exact reasoning for the save on the part of the developers is unknown to me, but the save entry is certainly not pointless because many game mechanics interact with spells based off of whether they allow a save or not and of which type. You don't need to worry about the save unless it comes up, at which point what to do with it will be clear. The pfsrd site suggests the save be editorially changed to 'none', since the '(see text)' clearly doesn't apply, but that's sort of up to your DM.
Yes. The summary text is just giving you a brief and inaccurate summary of the spell effect. The summary text does not dictate the effect of the spell or its behavior.
"A creature that successfully saves against a spell that has no obvious physical effects feels a hostile force or a tingle, but cannot deduce the exact nature of the attack." (Page 216 core rules)
No, that's not how spells work. The 'range' entry has nothing to do with how a spell works after it is cast, unless a spell specifies otherwise. For spells with a target, it merely limits what targets are valid. This text does not let you run around outside the spell's range with your stolen body, you could already do that by default. Furthermore, line of sight and line of effect also only matter when casting spells. You can take over a guard, run the full duration of the spell in a straight line, closing lead blast doors behind you every 10 feet, and then get thrown back to your body when the spell ends unless something extra prevents that from happening (in which case you die). The only time range matters in this spell after it is cast is in the case that the host body dies.