Potato Head,
autorotation is what you do when the engine quits on you. You basically enter a controlled descent, much like gliding with a fixed wing aircraft, where the air streaming upwards through the center of the rotor disch turns the rotor enough to provide some lift.
The easiest way to picture how it works is by drawing cross sections of the rotor blades and the freestream velocities in relation to the blades. A constant component upwards and a small horizontal component from the rotation at the blade roots but a large one at the tips. At the blade roots, the lift vector (perpendicular to the freestream velocity) will give a larger forward component than the rearward component of the drag vector (parallell to the freestream velocity) making the rotor turn.
When the engine dies, you push the collective lever down (setting the rotor to "fine pitch") and push the nose down. As you approach the ground you pull the nose up and pull the collective back up. Ideally, you want your skids/wheels to touch down with zero IAS and with the rotor coming to a stop. The energy stored in your forward momentum and in the rotational momentum of the rotor disc is what slows down your descent.
Cheers,
/ft