What happens

In this demonstration one of the Force members lies on a bed of nails. Another Force member breaks a cinder block on their chest.The person on the bed of nail stands and shows that their back is uninjured by the demonstration.

This is a highly dangerous demonstration undertaken by experienced performers. The equipment is specially built for this demonstration. DON'T TRY THIS AT HOME!

How it happens

How is the person on the bed of nails unharmed during this demo?

They are not injured because the nails are spaced evenly apart and their weight is distributed evenly across the nail heads. When the cinder block is broken, they are not hurt because the block absorbs the blow and distributes the force equally across their body.   Further down force on the nails, caused by absorbing the blow from the hammer, is again distributed across the nail heads.

Vocabulary

force: an interaction that when unopposed will change the motion of an object. It can cause an object with mass to change it's velocity. 

inertia: the resistance of any physical object to change it's state of motion. Bodies at rest stay at rest. Bodies in motion stay in motion. 

Physics principle demonstrated: inertia