Bigby's Disrupting Hand

A small, spectral human hand springs into existence before your target.
It hovers in the air, ready to deliver a backhanded blow.
Like Bigby’s interposing hand, this spell creates a magical construct in the form of a human hand.
The hand targets a single opponent of your choice within range.
You choose this opponent as you cast the spell.
You can target a different opponent with this spell by taking a move action to do so.
When the subject of Bigby’s disrupting hand attempts to cast a spell, the hand punches him.
This attack causes no damage, but it forces the subject to make a Concentration check with a DC equal to this spell’s save DC.
If the subject fails the check, the hand’s blow ruins his casting attempt.
The hand is about 1 foot long.
It has hit points equal to half your full normal hit points, and its AC is 20 (+4 size, +6 natural).
It makes saving throws using your total save bonuses and takes damage from spells and attacks as normal.
Focus: A soft glove.