Horror of Horrors!

"I'm right-handed," you tell MagnifiCAT.
"Two right-handers? Then we shall have to leave it to chance." MagnifiCAT puts down the cleaver and plucks a large, shiny coin from midair. "You are tails," he tells Will.
He tosses the coin. It rises slowly, catching the spotlight as it spins. Then it settles back into his outstretched hand.
"Tails," he announces. He picks up a piece of wood with a semicircle cut out of it and stands it on the chopping block. Then he forces Will to bend over and lay his neck on the cutout.
"Hold him just like this," he orders you.
You glance down at Will. His forehead is shiny with sweat, his eyes filled with terror.
You put one hand on your friend's shoulder and the other on the back of his head. "Like this?" you ask.
"Perfect," MagnifiCAT tells you. "Now listen. This trick relies on absolute precision. No matter what happens, you must not allow your friend's head to move. If there is even the slightest movement, the result could be deadly. Do you understand?"
You stare at him and shiver. "Yes, I understand."
"Good." He turns to face the phantom audience.
"Ladies and gentlemen," he booms. "The priests of ancient cultures discovered the occult art of keeping the most important parts of the body alive even after death. That knowledge was lost many centuries ago. I, MagnifiCAT, have spent many years rediscovering it. Today you will see the result. Right before your eyes, I will cut off our young volunteer's head at the neck. Yet the head will still live!"
He picks up the shiny cleaver.
What? It must be some kind of trick.
NO! Zack gasps. He twists to face you. "Help me!" he begs. "Don't let him cut off my head!"
"Hold still and don't worry," you reply. "It's just a trick."
You wish you felt as sure as you're trying to sound.
You glance at MaginifiCAT. His face and hands are pale white with a hint of green and his eyes are like black holes with glowing coals at the bottom.
"As soon as he starts to swing, pull me out of the way," Will whispers. "Please!"
"You heard him," you reply. "If I let you move, you might get hurt."
"I don't care," Will insists. "I want to keep my head!"
What do you do?