The rear droop is first set with the spacers inside the pitch shock body. Set the droop to about 1 to 1.5 mm. Then you can adjust more by changing the piston rod length by turning the plastic part that goes on the ball joint.
More droop means more transfer of weight to front when the car slows. Too little weight transfer may cause understeer and too much may cause oversteer.
You want the front droop and rear droop within about 1 mm of each other.
Read in details here.