When a helicopter’s engine dies, it doesn’t just fall out of the sky like a brick, despite what some people believe. It falls out of the sky like a very aerodynamic brick.
Under normal flying conditions, the engine turns the main rotor, which forces air down, and the helicopter up. Well, that’s the gist of what’s happening, anyway. If your engine dies, the reverse starts happening: air coming up through the rotor disk causes the rotor to spin. This actually generates a certain amount of lift. Not a lot, but enough to (hopefully) get you on the ground without damaging the aircraft too badly.
The maneuver itself is pretty simple. As soon as the engine dies (whether because of an actual failure, or because you’re practicing and cut throttle), you bury the collective, to get as much energy into the blades as possible. The inertia of the blades turning is where you store your energy. At this point, you start falling. Fairly fast. About 1,500 feet per minute (a normal descent rate is around 500, or less, depending on a few things). Really, that’s it: you’re autorotating. The fun comes in at the ground.
Your objective, just as with a normal landing, is to reach zero vertical speed and zero horizontal speed (unless you have room to do a running landing) at the same point, then transition to a hovering auto, then set down. The general procedure is largely the same as with a normal landing, too, you just have to be careful about how fast you’re coming down. Among other things, your blades can actually start moving too fast and be damaged, so you’re constantly checking the RPM gauge, checking your descent rate, looking outside, and so on. What’s really fun is practicing these, however.
You don’t normally practice what’s called a “full-down” auto, where you actually ride the autorotation all the way to the ground. You come down and do a power recovery, instead. What this means is that as you get close to a normal hovering point above the ground, you bring the throttle back in and come to a hover before touching down. This is actually far harder than doing a full-down auto, for the simple reason that you’re transitioning, very quickly, from autorotative flight, through normal forward flight, to hovering flight in a very short period of time.
In actual control terms, you have the collective very close to full down for most of the maneuver. As you get close to the ground, you flare, meaning you pull back on the cyclic to slow your forward speed, and start to raise the collective to slow your descent rate. Neither of these actions require you to change the pedals, since they act purely to counteract the torque of your main rotor, which doesn’t have any if the engine’s not turning. However, as you bring in the power on your engine, the rotor starts producing torque again. This means that you have to bring in some right pedal. Then you increase engine throttle up to operating speed, requiring left pedal. Then you start raising the collective, and modulating the throttle to stay in the green, and you end up with even more left pedal. Going from basically full right pedal to a large amount of left in a very short period of time, while also flaring the helicopter, and smoothly pulling collective. Once again, the helicopter requiring constant adjustment of every control at the same time.