Taking This in Steps

• A creature is in a stance until it takes a swift action to change its stance.

• While in the 1st-level Shadow Hand stance child of shadow (ToB 76) "[i]f you move at least 10 feet during your turn, you gain concealment against all melee and ranged attacks until the start of your next turn. You also gain the standard benefits of concealment, but you cannot use this stance to hide in plain sight...."

  Here's all about concealment

  Concealment Miss Chance
  Concealment gives the subject of a successful attack a 20% chance that the attacker missed because of the concealment. If the attacker hits, the defender must make a miss chance percentile roll to avoid being struck. Multiple concealment conditions do not stack.

  Concealment and Hide Checks
  You can use concealment to make a Hide check. Without concealment, you usually need cover to make a Hide check.

  and that's different from

  Total Concealment
  If you have line of effect to a target but not line of sight he is considered to have total concealment from you. You can’t attack an opponent that has total concealment, though you can attack into a square that you think he occupies. A successful attack into a square occupied by an enemy with total concealment has a 50% miss chance (instead of the normal 20% miss chance for an opponent with concealment).

  The stance child of shadow doesn't give the adept total concealment or make the adept invisible; those are different effects. The stance's effect just gives him an opportunity to use the Hide skill to hide and a 20% miss chance versus melee and ranged attacks.

  The DM must decide what movement the stance child of shadows requires. I would argue anytime a creature has gone from one square to a different square it has moved, but stricter interpretations--such as limiting them to actual movement modes (i.e. burrow, climb, fly, swim, and land Speed)--are equally reasonable. Jumping should qualify under either as part of a creature's land speed.

  If movement places a creature in weird position, the standard means to determine if the creature has concealment apply:

  To determine whether your target has concealment from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target’s square passes through a square or border that provides concealment, the target has concealment.

  When making a melee attack against an adjacent target, your target has concealment if his space is entirely within an effect that grants concealment. When making a melee attack against a target that isn’t adjacent to you use the rules for determining concealment from ranged attacks.

  This means while it might be weird and unexpected for the defender to find the attacker suddenly on the ceiling above him--and might entitle the attacker to a bonus to attack rolls for having higher ground--, the target will notice him unless the lines drawn don't allow it or another effect intervenes. It's D&D, and folks actually do check ceilings for enemies.

• A creature then takes a swift action to change his stance from the stance child of shadow to the 3rd-level Shadow Hand stance assassin's stance (ToB 76) is eligible to inflict +2d6 sneak attack damage (PH 50).

  The adept using the stance assassin's stance is eligible to inflict that sneak attack damage if he meets sneak attack's conditions: Is the target denied his dexterity bonus to Armor Class or flanked? If he is either or both, then sneak attack damage applies. As @TheXenocide noted, by itself, although counter-intuitive, neither concealment nor total concealment meet sneak attack's criteria so another method of meeting the criteria must be used. I suggest blinding the creature or turning invisible.

  This question addresses using concealment or total concealment in combination with sneak attack in greater detail.

Use the spells delay death and beastland ferocity

The 4th-level Clr spell delay death [necro] (Spell Compendium 63) says

The subject of this powerful spell is unable to die from hit point damage. While under the protection of this spell, the normal limit of –9 hit points before a character dies is extended without limit. A condition or spell that destroys enough of the subject’s body so as to not allow raise dead to work, such as a disintegrate effect, still kills the creature, as does death brought about by ability score damage, level drain, or a death effect.

The spell does not prevent the subject from entering the dying state by dropping to –1 hit points. It merely prevents death as a result of hit point loss.

If the subject has fewer than –9 hit points when the spell’s duration expires, it dies instantly.

The spell targets 1 creature within close range as a standard action and has a duration of 1 round/level.

The 1st-level Drd spell beastland ferocity [ench] (Spell Compendium 25) says

The subject becomes such a tenacious combatant that it continues to fight without penalty even while disabled or dying. While between –1 and –9 hit points, the creature gains a +4 enhancement bonus to Strength. If the creature is reduced to –10 hit points, it dies normally.

The spell targets 1 touched creature as a standard action and has a duration of 1 minute/level.

Both are low enough level to fit in wands, and some prestige classes can even cram them into potions.

A legalistic DM may kill the creature affected by both spells when the creature hits –10 hit points anyway, as per the spell beastland ferocity, but the effects of the spell delay death should alter what would otherwise be dying normally for the affected creature. Nonetheless, before whipping out this strategy at the table, I strongly urge first clearing it with the DM.