Knocking down helps for several reasons. #1 is it releases tension from the center and #2 it also helps the paint keep flexibility while applying pressure from behind.
Now since I offer personal training and sell a detailed PDR Line Board manual, I can’t tell you exactly how and what tools I used to remove the dent. What I can tell you though is that this type of dent will take a lot of patience and accurate pushes to get your reflection straight as possible.
If you are interested in PDR training, please email me and I will send you some more info. I train in real situations at dealerships, body shops and with retail customers.
The whole thing about PDR training is the learning experience and environment. My training is not a classroom situation and not a routine. If it was a routine it would only benefit myself and not yourself the student. Every situation is different and that is the only way I feel a student can grasp PDR at a much better pace.