Strange, they never asked those condemned to actually utter some sound or mimic something, isn't it? Well, that's because residual muscle twitch is not controlled and is not conscious. Rapid drop of blood pressure renders the person (well, their head, in that case) unconscious within a couple seconds and it is downhill from there.

> Well, that's because residual muscle twitch is not controlled and is not conscious

And because making sounds without lungs attached to your throat is kinda hard