Changing BodyVelocity.P doesn't do anything?

I want to know what BodyVelocity.P exactly does. The wiki says it determines the magnitude at which the force on the object is applied, so I conclude that the smaller P is, the slower it accelerates, and the larger P is, the more it accelerates. But this doesn't seem the be true, as when I change P, the brick movement doesn't change.

So when I use lua to change P, or when I do it in studio, nothing happens. What is P?