What is a 'high-side' crash on a motorcycle and why is it particularly dangerous?

1. When the motorcycle falls gently onto its side while parked on the kickstand
2. When a sliding rear tyre suddenly regains grip, snapping the suspension upright and violently catapulting the rider over the top of the motorcycle
3. When a rider takes a corner at exactly 20 km/h
4. When riding over an elevated speed bump