The clutches always grab. They slip when there is a certain difference in torque between the two wheels (the amount is in the FSM, 200 ft-lbs or so I think). The idea here is that if one wheel loses traction, with the clutches starting to slip, then you know that you have at least that amount of torque being applied to the wheel that still has traction.
I find the name "limited slip" to be a bit misleading. It can actually slip an "unlimited" amount. It's just that the friction between the clutches will keep some amount of torque applied to the wheel with traction no matter what (unless the diff is broken inside or whatever).