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Autorotations are used in helicopters to perform power off landings from altitude in the event of an engine failure. During an autorotation, the main rotor is not driven by a power plant, but by air flowing through the rotor disc bottom-up (imagine a windmill) while the aircraft is descending rapidly

My understanding is that autorotation is used to store kinetic energy in the blade rotation, then at the last moment, the pitch of the blades is increased (decreased? reversed? not sure) to slow down before landing. Surely I could be wrong.

The simplest way to put it:

imagine the main rotor (the one on top) spinning in it`s normal direction. Start from the ground... wait until the speed is at the speed you need. You pull up the collective, which changes the pitch of the main rotor to give you lift. Simple enough.

Now, turn the situation around; you lost the engine(s) so you have no power. If you have forward airspeed, you can use the forward airspeed to your advantage. If you don`t have forward airspeed, you can still do it.

The first thing you need is enough altitude above ground level... then depending on your angle of attack (forward airspeed plus tilt of the nose of the helicopter), you make necessary adjustments... push your collective all the way down to reverse the pitch of the rotors so that while you are falling, it will actually speed up the rotors, while you are falling, and also act as a makeshift parachute to slow your decent down.

That`s the easiest way I can describe it :/ The rest all depends on HOW (what type situation) you started (forward motion or hover). If you started with forward motion, then you will need a little room (and pray for no power lines to get in the way).... and if you happened to be in a hover, you need almost no room, depending on the total size of the chopper (I wouldn`t mind losing the tail boom hitting a tree or something as long as the pilot/passengers/innocents in the area are safe).

I hope I helped and not hurt ;)

Oh, more thing... Obviously in this situation (2 news choppers), there was nothing the pilots could have done after the initial contact. From the picture, it looked like both choppers lost at least the main rotor, which is the only way the AR works.