This may lead to an ankle sprain or to a more serious condition called snowboarder's ankle. This is in fact a fracture of the lateral process of the talus bone in the ankle and it presents like a severe sprain. It may need a CT scan to diagnose it and an operation to fix it. If left untreated, it can lead to the early development of arthritis and long term disability. Whilst on the board with both feet attached in the bindings, any twisting force occuring will twist the whole body as one - both legs and trunk must therefore move in the same direction.
Snowboards tend to be shorter than skis which reduces the lever arm of force produced by any twist.
Although knee injuries are rare in boarders, it is on lifts that they occur. Well, the lead leg is still attached to the board whereas the trailing leg is not. Head injuries are usually the consequence of a fall. Most head injuries are thankfully minor in nature. However, with increasing force of impact the risks increase and can include death or long term disability.