Backspin

While riding forwards, slowly turn the upper body 90 degrees left (or right) relative to the wheel's direction.
Quickly twist the upper body 180 degrees (the action), causing the wheel to pivot on its vertical axis 180 degrees in the opposite direction (the reaction).
Recover from the 180 degree wheel snap by slowly returning the upper body to a normal riding position from its 90 degree right (or left) position.

Notes:

A backspin is defined as riding forward, quickly rotating (snapping) the wheel 180 degrees about the vertical axis, and then riding backward. This must all be done without any perceptible change in speed or direction (of at least the rider's upper body [except for twisting which is required to perform the skill]).

Ideally the path of the wheel when doing a backspin is a straight line, but in actual execution, the path contains a slight "dip" where the wheel changes directions. So the actual path is more like the line below, except the "dip" (\/) is actually two matching smooth curves which come to a sharp point at the tip. The "dip" can deviate from a straight line by as much as 0.5 meters, but should be much smaller (Ask your examiner how small).