Diffs arent designed to slip. When your on the track it would sound almost like a duck quack when it slips. Tighten it down pretty darn snug, and then back it off just a tiny bit. run it on the track and if the diff slips at all tighten it a bit. You dont want to crush the balls or anything . but it should be tight. The slipper is where you adjust for the track conditions. Generally it should slip for about 1 foot from dead stop under full throttle start. If you hear the slipper slipping OK. But if it "quacks" thats your diff and the gear will melt very fast!
When I rebuild my diffs I set them tight then back off until it feels decently smooth.
Then I hold the tires and spin the spur gear with my thumb. The spur/slipper should spin befroe the diff.
Usually I want to hear the slipper slip on track for a foot or two from a dead stop. But it should usually be quite once up to speed so mine deosn't slip very often. It rare that I'll set it to slip out of every slow corner.
I believe the answer to this question has to do with what kind of surface your running on. The loser the track(less grip you get) surface the loser you can set your diff. The grippier the surface (more traction) is the tighter you will set your diff. The difference between the two....lose and tight diff setting could be as much as almost a full turn or as little as 1/4-1/2 a turn on your diff settings. This is a key to getting your car to turn with out wanting to spin out, but you have to make sure your slipper is set properly so you don't burn up your diff. Like stated earlier, diffs aren't designed to slip so getting these settings right is crucial to your diff lasting long and a consistency in feel race to race. Learning this might take a little time but will be a huge help to being able to drive with confidence. Also if you driving a 2wd car this will really help you keep the car straight under hard acceleration, and accelerating coming out of turns. Check the diff by holding on to one tire and keeping the spur gear and motor from turning with one hand and try and turn the other tire with the other hand. If you can turn that tire with out letting the motor and diff turn then its to loose. Tighten the diff a 1/4 turn or so and try it again. Once the diff is set then set your slipper, basically you want the slipper to slip before your diff. A good foot or so of slippage is a good starting point to shoot for from a standing start under hard acceleration. Remember though that these slipper setting need to be done on the track surface that you race on, also in higher traction situations you want less diff action so you will tighten the diff a bit to obtain more of a possi track setting...or to slow down the diff action.