Misti, since I gather from one of your other posts that you're an instructor and I'm a straight noob, I'll give it a shot

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A high side is losing rear wheel traction while cornering and being ejected from the bike when the bike suddenly gains traction and rights itself violently.
How to prevent one? Gee, short of spending some time on a dirt bike to learn to drift the rear wheel, I'm not sure considering I'm sure there's a lot of practice and feel involved. I suppose staying on the throttle to keep the rear from gaining traction too suddenly and steering "into the skid" as they say. The fact that staying on the throttle is so counter-intuitive is why I think it'd help to practice this one in the dirt. Also, it looks like the rider in this situation could have had his body moved off of the bike more to allow it to remain further upright. Looks like quite an extreme lean angle.
Me, I corner like a grandpa, so I don't have to worry about it.