I noticed a clause in Bigby's Hand that has me pondering…
The hand lasts for the spell's duration, and it moves at your command, mimicking the movements of your own hand.
Here is the situation:
Bob is a 9th-level wizard. He is carrying his Wand of the War Mage +2 in one hand, and his other hand is open. Bob does not have the War Caster feat.
Bob casts Bigby's Hand and uses it to grapple a foe.
Does Bob still have a "free hand" to cast a non-concentration spell the following round?
Bob doesn't want to drop the Wand, as it is his spellcasting focus and he wants the bonuses. He doesn't have the War Caster feat, so he cannot "perform the somatic components of spells even when [he has] weapons or a shield in one or both hands." But per the spell description, Bigby will mimic the spellcaster's hand movements. So if Bob makes "a forceful gesticulation or an intricate set of gestures" with his other hand, does that mess with the grapple?
I've never seen a DM rule that Bob was out of luck with somatic spells, but I'm looking at this from a RAW perspective.
On the other hand, I can't help but picture the poor creature that's being grappled turned sideways while Bob puts his thumbs together to cast Burning Hands.
Best Answer
Yes.
The solution relies on the description that you quoted (emphasis mine):
Per my reading, this means that the Hand moves and mimics the gestures of the caster's hand when they command it to do so: during the casting of another spell, a caster is not commanding the Hand, hence the latter does not replicate the gestures of the caster.
Moreover, the description of the Grasping Hand says:
and does not contain anything similar to "while the hand is grappling the target, one of your hands must be clenched".
Suppose that the Hand has been evocated in a previous turn: the caster's actions order may be the following:
The "implicit command" reading is suggested by the Interposing Hand option:
The description explicitly states that the hand moves autonomously for interposing itself between the caster and the target, without requiring the latter to mimic the movements of the Hand. The "implicit command" is to adjust its position with respect to the target in order to defend the caster.