Downshifting is the process of selecting a lower gear, generally on the approach to a corner. Downshitting is spinning a RWD car while shifting because the rear wheels locked up. There can be multiple sources of downshitting. The simplest is to miss a gear change and put the car into 1st instead of 3rd.
More commonly downshitting is caused by people who habitually engine-brake (i.e. use the friction of the engine to slow the car down). In racing, this is wrongity-wrongity-wrong. Your brake balance should be set up with pad compounds and prop valves to provide the ideal braking balance WITHOUT the engine. As soon as you put the engine into the equation, you are changing brake balance. In a FWD car it’s not a big deal. It just puts more braking power on the front of the car, which paradoxically increases your stopping distance because your rears are doing less work. But in a RWD car it is a big deal because once your rear brakes overpower your front brakes, you will spin, even going in a straight line. Do that in a corner, and you’ll snap around so fast you’ll think you were hit.
Intentional or not, the source of downshitting is releasing the clutch pedal when the vehicle is going one speed and the driveline is going another. The driver below gets his foot caught under the throttle and can’t get the revs to match. I’ll bet if he had a do-over, he would have kept the clutch in and coasted around the corner.