What is the vortex ring state (also referred to as settling with power?

Something Remote pilots of rotor type unmanned aircraft should use particular caution. Happens when descending in a vertical straight line. The turbulent downward airflow can disrupt the normal production of lift by the propellers as well as cause problematic air circulation producing vertices. Recovery from this state of flight requires forward or rearward motion - counterintuitively, the addition of power to arrest the descent only make the situation worse