The word "hand" is a metonym for the power of the owner of the hand. Possible meanings are 1) "only if someone stronger than he forces him to let you go" or "only if I force him to let you go," or 2) "not even if I force him to let you go." (See: [[rc://en/ta/man/jit/figs-metonymy]] and [[rc://en/ta/man/jit/figs-idiom]])