When I get normal contractions, they are like trying to breathe out, but when I get contractions on exhale, they are trying to breathe in. They happen more in dry negative statics, and hardly ever in the pool. They feel much worse than normal contractions, which is why I don't do negative statics very often.
It also scares me to have to actively stop myself from breathing in water. If I blacked out, it would probably be the first thing I do.