Flapping your glider
This maneuver has limited uses. Here is a YouTube of a pilot attempting to land at a thermic LZ while a mild dust-devil was blowing through. Did it work? You decide.... Everyone can see that the pilot might have spun or stalled his glider.
Master pilot and instructor, Chad Bastian, has this to say:
I have used flapping successfully many times, but it needs to be used with caution. Flapping the wing on your way down creates a very inefficient glider and significantly degrades the glide. It can be used as long as the pilot is very aware of the stall point and does not cross it.
It works particularly well on a ridge site when coming in to land and the ridge lift keeps lifting you up, but can be used successfully in any landing situation where you do not want to be lifted up. The biggest drawback in anything but smooth air is spinning or stalling the glider because the air is not fit to be flown in! (Dust devils, etc.)
The bottom line is that if you are higher above the ground than you would like to free-fall, don't do it.