The theory is simple-- on loss of power, by establishing a negative pitch angle (heli upright) before your hs decays below controllability, you can use the airflow as the heli descends to maintain headspeed (the air drives the rotors instead of the other way around). The autorotation gear on driven-tail helis disengages the motor drag from the rotors, and the mains rotation continues driving the tail so you have yaw control. If you maintain the energy in the head during descent, you can flare at 3-5 feet above the ground by increasing collective to a strong positive angle, converting the remaining energy into lift to cushion the landing.

In practice, the descent is very rapid, and it takes a good amount of practice to judge the flare and to get the timing right so you don't stall (with no remaining energy) above the ground, or flare too late and just make a hole. You usually practice autos by reducing the motor to idle and flaring far above the ground so you get a feel for it but still have altitude to restart the rotors. It's tricky to get it right. It's even tricky to practice without crashing, for that matter.