Rey if you are doing PP the right way, first understand what your dog can do, then figure out what it can't, then go from there. A good PPD should have balanced prey and defense drives. Some high in the defense. Also understand that some lines are SLOW to mature even at 2 years of age!
If you take the dog faster than it can mentally handle it, you will screw it up. Find someone who understands the concepts behind bite development and GS lines. Not all soccer plays make it to the national team, not all cops eat donuts, and not all German Shepherds can do REAL bite work, in prey yes...when the stuff hits the fan, no. This goes for any breed.
Pohraniční Stráže)... oh crap there is your answer!
Not all cops eat donuts? :-o8-[