After confirming the suspect was no threat, the officer started uncuffing him.

uncuffing 是动词 uncuff 的现在分词形式。uncuff 由前缀 un-（表示"去除、解开"）和名词 cuff（"袖口、手铐"）组成，字面意思是"解开手铐"。uncuffing 在这里指"正在解开手铐"的动作。