My shorter Cliffs Notes type version (corrections welcome):
On the track, the usual high-side is coming out of a corner and getting on the throttle too much. The back tire starts to spin and slide sideways, then suddenly gains traction before the bike is upright, throwing you off when the bike suddenly straightens itself. Another possibility is shifting down too many gears and letting the clutch out too quick in a corner, so the back tire starts to slide sideways.
On the street, the usual high-side is braking hard, the back tire locks up and starts to slide sideways, then suddenly the back tire gains traction and throws you off. I did one of these in 1984, from my back tire sliding on wet railroad tracks then getting more traction on the wet pavement after the tracks. I estimate my butt got about 6 feet above the pavement before its hard landing. I had trouble getting out of bed the next morning, from all of the bruises. This was when I was 25 years old. At 50 years old, I was riding a bicycle and my butt fell from about 5 feet high, and I missed 3 weeks of work from the bruising and pulled muscles.
On dirt, the usual high-side crash is sliding the back tire sideways then catching it on a rut, rock or root. The lighter dirt bike does not throw you off with as much force as a heavier bike. I did the most memorable high-side in about 1972 racing bicycles- the pedal from another bike got caught in my frame and both of us flew over the handlebars when the bikes tangled, slid sideways, and flipped into the air. I landed on a rock chest first and broke two ribs. After this I started to learn to land feet and butt first, then slide on my back.
To prevent a high-side, start with lighter and less powerful bikes. A new rider greatly reduces their crash chances with a lighter 250cc or 500cc 2 cylinder street bike with a narrower back tire, or a 650cc single cylinder, instead of a 650cc twin or 600cc four cylinder.
Newer sportbikes often have traction control and a slipper clutch, some have ABS braking, and you learn safer corner entry speed, safer lines, and how much throttle is safe just after corner apexes.
On the street, if the back tire stops spinning during braking you either keep the back brake locked until you stop, or with more skill you get on the throttle after braking to keep the back tire spinning slow. I did this about 5 years ago when a truck turned left out of a driveway in front of me, and I needed to do a 35mph sharp corner to the right to miss the driver door by about a foot, braking hard, doing a drift, then getting back on the throttle some for a controllable slide. I did not use the clutch like the expert below- more skill is needed to control the clutch and back brake at the same time.
This expert below shows how he drifts in corners. It does not show the hundreds of crashes, probably mostly on softer dirt, that he did while learning this skill: