The slipper is fitted to the spur gear. The tighter you screw it down the more instant torque goes to your drivetrain. This way you can wheelie on command but it stresses your drivetrain more. Therefore it is better to loosen it quite a bit to protect your drivetrain from shocks coming from landing after jumping for example.
IB 3800 should be a good fit and are similar to my GP 3700.