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.