There's a required amount of nose down on the pinion in order to absorb the shock load of hard race launching. There is stock spec around 2*down and then race of 4-8* down. If the pinion is almost straight to the Driveline stationary with no load then when you hit the gas and load it it's going to put it nose up and win the Springs up and the Springs wound up will then resist and unwind breaking the tires loose and then repeating over and over
