Why can't Lennie let go of Curley's hand in Of Mice and Men, Chapter 3?
It seems as if Lennie wants to let go of Curley's hand, but cannot. If he attacked Curley out of blind obedience to George, he would have been able to let go of Curley's hand when George told him to let go.
When Curley starts punching Lennie, he's too scared to fight back. At first, Lennie doesn't even raise his hands to protect his face. When Lennie does cover up, Curley attacks his mid-section. George yells multiple times, "Get him, Lennie!" It takes this many times because Lennie is panicking. Also, George has told Lennie again and again not to fight and to avoid Curley. Lennie was conflicted and panicking. That panic continues when Lennie finally obeys George and fights back.
Just as Lennie doesn't know his own strength in these, for him, terrifying situations, he also doesn't know what he's doing. And just as it took multiple commands to "get him" it takes multiple commands to get Lennie to let go of Curley's hand. Lennie has always obeyed George but in moments like these, Lennie is almost comatose and doesn't realize what he's doing:
But Lennie watched in terror the flopping little man whom he held. Blood ran down Lennie's face, one of his eyes was cut and closed. George slapped him in the face again and again, and still Lennie held on to the closed fist.
In such situations, Lennie doesn't know his own strength because he doesn't know what he's doing. The same thing occurs with Curley's wife. Lennie panics and doesn't realize what he's doing. Lennie does attack Curley when George tells him to, but it takes multiple commands because Lennie is in a panic. For the same reason, it also takes multiple commands for Lennie to let go. In these moments, Lennie can not mentally make sense of the external situation.