The WWII Project uses both …. I think.
A unit can voluntarily go to ground after movement.
A unit can be “hit” and either automatically go to ground or go to ground if it passes a discipline check (bad things happen if you fail that check).
If a unit that has gone to ground has a leader or hero attached, it can get right back up and keep moving at its next opportunity. Otherwise it must pass a discipline check before it will rise from the ground and do “whatever”.