I really don't see how people say that is painless. Thats equall to us having our heads strapped down and our lower body pulled the other way until our necks or spines snap. Painless? Yeah right.
But yeah, the way some people do it is hit the mouse right on the neck with a hard pole-like object, then holding the pole there and yanking the tail.
I really dont recommend this, because obviously it causes the poor thing pain. It's probably more painless just for the snake to strike it and quickly constrict it.