Trust no one—take the handcuffs off a prisoner while under your strict physical control.
Have the arrestee assume the standing handcuffing position (legs wide apart, feet turned outwards) while possibly controlling him with a rear double-twistlock.
Remove the cuff of the hand that you do not have control over.
PRO TIP: Removing the cuff on the seized hand first will leave him with a free hand bearing an open cuff, one which could be swung at you.
As his hand is released, close the handcuff, gripping the chain portion of the handcuff, so the chain passes between your middle fingers [Fig. 4-13.18] or by inserting your fingers into the closed cuff to grasp it.
Keep his newly-freed hand in his lower back (not in a loaded position on top of his head!).
Step away from him and stay in his blind spot to release his second handcuff keeping a grip on the handcuff, or attaining a bent wristlock or twistlock on his straight arm if need be.
You can then safely back away from him (use your elbow against his back to prevent an unexpected headbutt).