(Someone correct me if I'm wrong - been around a lot of Losi's and going on memory here)
The outer disk of the slipper clutch is generally what the spur bolts to - you should see two plates with a fiber wafer sandwiched between them. It is adjusted by the spring-loaded bolt protruding from the end of the topshaft.
Loosening the slipper slightly does two things: diverts hard accelleration force from the tranny where it can do damage, and more appropriately allows the SLIPPER to slip instead of the tires. If you're constantly spinning out coming out of corners or on hard accelleration, or burning up diffs frequently, this WILL help.
When you first loosen it up, the truck will just sit there or barely move when you gas it. Begin snugging it down and listen to the engine as you spool it up - you should "hear" when it fully engages. Basically if you hammer it from a dead stop, the wheels should not spin but you should hear the engine wind up. As if gets moving faster and the tranny side catches up with the RPM, it should engage.
I generally get them to fully engage within 12-15 feet on electrics, 20 or so with nitros.
The goal is to be able to get into and out of corners and across slippery sections of track with little or no wheel spin when you get on it. It's why the slipper is also called a traction control device.
Too loose and the slipper assembly will overheat. Always re-adjust it on each surface, as it will slip more on high traction and less on low traction.