With helicopters, Ground Effect phenomenon occurs when hovering at or below half the rotor diameter. It is essentially a cushion of air generated by the rotor, which the ship rests on. For this reason, hovering in Ground Effect takes less power than hovering higher - due to the air cushion, which is providing support. The ship gets "twitchier" while in Ground Effect, as well. Hovercrafts use this as well - a hovercraft never lifts out of ground effect, and thus needs much less energy to maintain its above-ground position.