The jump in the air should be the result or throwing your bodyweight into the break. After having thrown your weight forward during the stroke, you redirect your momentum upward during the follow-through so as not to fall forward onto the table.
However, having seen their breaks many times on TV, I don't think any of them are really doing that. I think most of them are jumping up in the air because they've asked men or other ladies with powerful breaks for pointers, or just watched to see what they do, and found out that powerful breakers jump into the air, so they started doing that thinking it would make their break more powerful, which is sort of like collecting speeding tickets thinking it will make your car faster.
-Andrew