Take your pick:
The bindings stay on during a "digger." The board stops and holds on to your ankles resulting in a whipping motion of your upper body where the result is a hard SMACK to your face. But...Your ankles and knees are safe.
Option 2: On a digger, the board stops and releases you. This allows your body to continue traveling and come to a less abrupt stop or slide. You can still get smacked, but it is usually less violent. The downside is a "one in one out" scenario where the board spins around and blows out a knee or ankle.
I like option 2...loose boots. But I think you'll find it 50/50.