Power,
Was wondering cause I had never heard of the phrase (among many).
If swingbuilder is talking about a slight tension on the inside of the thighs/knees during the stride until toe touch, I understand his point but don't know if all hitters need to do so. I always had a very slight tension (inside knees/thighs) during the stride in an effort to stay somewhat centered and avoid drifting my weight forward over my front foot...but it stopped at toe touch, and I always viewed it as a personal quirk. Can't say I would advocate/teach actually swinging/hitting with that tension. JMO
A beachball? I would pull a hamstring if I tried.