As he powered thru the corner, probably getting onto the throttle a touch early cos he could see the other bike pulling away, his tail end wheel started sliding out (probably cos he had a touch too much throttle wound on) so he steered into the skid (as you should!) but he didn't back off the throttle enough & the back wheel slid ALL the way out - resulting in a Classic Low-side prang!!
Whatever causes them, a Low-side prang is when the rider comes off the Low side of the bike/slide, & is often more 'pride damaging' than much else; while a High-side prang is when the rider goes over the bike the long way, ie, on the High side, & those types of prangs are far more likely to result in fairly significant injuries, especially if the rider tries to break their fall with their hands &/or the bike slams down on top of the rider! :shocked: Either way, they aren't great & are best avoided!