The rudder stalling is the result of the broach caused by a spirited gust or if sailing DDW the boat can start to rock from side to side and the rocking can get more extreme to the point of losing stability where the boat rounds up and then becomes pinned. This can result in a gybed broach. Though I have experienced numerous broaches from these two causes, I have never had a gybed broach.I always thought when you broach while sailing you lose control of the boat because your rudder has lost its ability to steer.
Agree, broach is caused by loss of rudder control, usually by stalling (equivalent of airplane wing stall) the rudder. I think this can also happen on some boats when they are on the ear so far that there isn't much rudder left in the water.I think for a broach to occur the rudder has to stall if not the boat will simply round up. Burying the rail is not a broach. I have raced on boats where burying the rail does happen quite often. On these boats the stanchions and lifelines are removed, mostly for sheeting but a tertiary benefit is there is less drag when the rail is buried.