In Planet Terror, Cherry Darling lost her leg and she was attached with a gun.
I am asking about the trigger.
How is Cherry Darling firing a gun with the leg?
Apparently by magic or telepathic command.
At several points the gun clearly fires without her hands being anywhere near the trigger mechanism. (Unfortunately, the actual picture of the shot blast is mid-frame and so very blurry) but the gun clearly fires by itself
You can watch the whole thing here.
By the end the has been upgraded from a modified M4 Carbine with a M203 grenade launcher attachment to a minigun!
The gun fires by electrical impulse. Much in the same way as "bionic" arms and legs are still usable by amputees, down to very touch-sensitive fingers, Cherry need only send an electrical impulse to her leg via her brain in order to fire the weapon. Think of it as, how does your big toe move? You send a signal to that muscle via your brain, and the toe moves. In Cherry's case, she sends a signal to move her toe, and the gun fires. You don't need to physically touch the trigger any more than an amputee needs to physically touch their bionic fingers to make them move.
And no, this is not explained in the movie. But it's a logical conclusion.