Just to help a few of you out
Skidding is where your rear wheel is not turning, due to the brake being applied.
Drifting/ backing in, is where a degree of traction is lost allowing the rear wheel to slide. But the wheel is still turning. You do this on a MTB by changing body position and unweighting the rear wheel.
The 1st is losing control, ie you've over braked to the point where you have lost traction and not corrected it by reducing pressure on the rear brake.
The 2nd is a method of cornering allowing to to take a tighter line. Requiring skill, and control.
Reamember, Leave no trace.