Vortex Ring State

happens when a helicopter (even a small quadcopter!) descends too quickly and its own downwash (turbulent air beneath the rotor(s)) envelopes the rotor(s). The machine basically runs into its own downwash. This may happen for instance due to a piloting error and leads to severe loss of lift.

Simply put, you go down too quickly and find that you start falling down and can't pull up! And Going full throttle doesn't help! The most straightforward way of getting out of VRS is to move horizontally to get out of the turbulent air as quick as possible.

Wikipedia article.
Quadcopter demonstration: