I spent 26 years flying helicopters and can tell you this phenomena referred to as "vortex ring state" is treated with great respect by helicopter pilots (It is also referred to as "settling with power"). I've read several very good posts that have addressed this issue, but after having read several recent posts of crashes I thought I would address it again. Getting in a settling with power situation where you are on final approach is extremely dangerous since your normal instinct is to add power and adding power only makes the problem worse. Simply speaking what happens is downwash from the rotors begins to recirculate up, around, and back down through the rotor discs and the multirotor sinks into the air mass. To avoid this phenomena avoid angles of descent between roughly 70 to 90 degrees with high rates of descent and slow forward airspeeds (particularly with a tail wind and near the ground). In some helicopters we avoid rates of descent that are greater than 800 ft/min (240 meters/min) below translational lift (approx 10-20 knots). I don't know what those numbers translate to for multirotors. If you do find yourself in this situation, try to increase your forward speed without adding power...again if you are descending uncontrollably, adding power will only make things worse.