I spent some time working on these types of cars this past summer. Using some sort of prop under the doors you don't want to open is probably one of those skills that was probably going to be lost, but now we've saved it due to this discussion!
Dropping the doors in a controlled fashion takes a bit of care. Numerous times we were trying to "let them down easy" when the paw slipped off the rachet and the doors opened completely, with a big bang. Even without a load, the doors themselves are fairly heavy. We had a standing protocol that anytime the doors were being lowered that everyone on the team needed to be in the clear anywhere around the car.
There is a rumor that if you try to open both sets of doors on one side of the loaded car, you can tip the car off the tracks, especially if you drop the doors in an uncontrolled fashion. One of the cars rebuilt over the last 20 years had apparently suffered that fate.
I'm somewhat surprised that there is no safety latch that would prevent the doors from opening while the train was in motion. You could probably derail many cars following a DBG that had a door latch fail. But I guess that never happened.
Bill Kepner