There's a balance between too much pressure and too little pressure. For years I've seen people post,
Just use a little more pressure than the weight of the machine
If you do that, you might be buffing out your car for decades. You need to
engage the abrasives with the paint in order for them to take little bites out of it, this requires more pressure than just the with of the machine. You don't want to push down so hard that the pad just barely rotates either as I've seen other posts. Push down hard enough to be applying firm pressure but still see your pad rotating at a good clip.
Mark your backing plate with a permanent marker so you can see when the pad is rotating and how fast or if it's stopped and just vibrating against the paint.
Remember, besides rotation speed other factors like pad choice, product choice and just as important, technique are very important when it comes to removing defects out of clear coat finishes.
The new generation PC's are also more powerful than first generation PC and are much better at keeping your pad rotating under pressure.
