It is when your body falls over the down-wind rail of the board, like a hinged knife folding closed, caused by losing your balance in the side direction. Feet stay with the board, upper body falls towards the water, and the foil wings rise up towards the surface as the board becomes on its side. This leads to falling off the side of your board, downwind, making contact with the wings that are rising up.
(Recognizing this loss of balance early, and 'jumping off' is usually effective at preventing this sort of contact. Attempting to recover balance is the problem- so just give up and accept a safe fall.)
But I'm not a believer that 'good technique' is a reliable countermeasure for injury. It should reduce frequency of injury, but uncontrollable falls are still a reality for experienced foilers.