Pay particular attention to the edges on your salvage yard test panels. If there is a sharper edge on one of the hoods (like a dip or a raise) try to get a feel for how quickly the rotary can cut through the paint on edges. It is better to get a feel for this on junk panels than on a customer's car. It can literally happen in 1 second in some situations.
The direction of the rotation of the pad is very important in these situations. You always want the rotation to travel off of the edge and not up on to the edge. When your pad rotation travels up on to and bites into a sharp edge it can cut through in a heart beat.
Below is a pic of a hood with the kind of sharp edge that I am talking about.
Note how the center panel in this hood is lower than the left and right sides of the hood. I will call the center the low side and the outer part on each side the high side.
On a hood like this you want to work the high sides first and "ensure" that while working on the left high side, the side of your pad that is rotating from left to right (the side farthest from the machine's handle assuming you are standing in front of the car) rotates off the high side in the direction of the low side. This ensures that the pad doesn't bite into the edge and remove all the paint from it.
Now lets suppose you are polishing the same left high side of this hood while standing in front of the car and you are running real close to or right on the edge as described above. If you would tilt the handle of your rotary down closer to the hood, this would cause the rotation of the pad to travel from the low side up onto the high side and it would bite into the edge. It would most likely bite in and kick the rotary back toward the center of the hood. The damage would have already have been done though.
As quickly as the pad would bite into the edge and kick back the polisher, it can cut clean into the primer that quickly on most softer paints.
I suggest that you try to find a hood with an edge like the one pictured above and play around polishing the edge, keeping in mind that this type of edge can catch you by surprise and be easily damaged. This is all tough to understand in just reading about it. If I were there with you I could show you very quickly what I am attempting to describe. Good luck and keep practicing.