Pinion angle helps with the hookup and launch. The rear end will want to rotate clockwise, if you are looking at the rear end from the side. By placing negative pinion angle on the rear you are already setting up the rear so that when it rotates clockwise and up, the drive shaft will line up with the yoke and keep from binding. You will also cause the front of the car to want to rise as the rear is acting like a giant fulcrum.