You want to apply firm pressure but no so much pressure the pad is rotating in slow motion. I've seen guys do this and preach this and if that works for them then that's good but I think it means you're pushing down so hard that it borderlines on mis-use of the tool and the panel is probably warping or bending under this kind of downward pressure.
Besides that experience from buffing out thousands of cars tells me you don't need that kind of extreme pressure to get good results.
I think if you watch this video carefully you can see how fast the pad is rotating, at least you can see enough to give you a ballpark idea as to how fast the pad should be rotating. How to Remove Swirls and Scratches using the Porter Cable 7424XP