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http://www.youtube.com/watch?v=BBScI_Mb-Gw
Max all up OH-6
After replacing the C-18 with a unit reconditioned by the local lads, the aircraft is put through some tests, including this one. The first bloke off (running away), is one of the engine mechanics! Fills you with confidence!!!
The aircraft is in ground effect. that is to say, it was hovering at a height where some of the downward moving air reflects off the ground and pushes back up at the helicopters blades creating an added cushion of air, a bit like a hovercraft. This is achieved at a height of 1/2 a rotor diameter.
So it's an efficient hover that requires less power to achieve. Above this height you require more power.
You would probably find a better explanation on google somewhere.