Brake hand down, brake hand back...by hip...by chest...whatever.
The key thing I get from this event is that once the redirect piece popped out, the belayer was basically confronted with a force equivalent to a large leader fall by a very heavy climber. And took this force directly onto her harness. In a downwards direction. It's hard to imagine a worse scenario when it comes to it being difficult to maintain control.
Glad everyone will be OK.