There's no external clamping involved with slip-resistant connection. A slip-critical connection is a subcase of a pretensioned connection. Both involve tightening the bolt to some specified level of tension; slip-critical means you're also concerned about the condition of the faying surfaces (the surfaces that are facing each other in the connection). It means that you're not relying on the bolt acting in shear (like a dowel) to keep the two plates from slipping past each other; instead you're relying purely on the clamping force applied by the bolt. A fully tensioned bolt and a well-prepared surface will do this very well, and there is no concern about the bolt or nut "working loose", as the same clamping force keeping the plates together is also keeping the bolt head and the nut against the plate and against the threads.
"Turn of the nut" is one means among several of getting the required tension; because of the pitch of the threads, a certain number of turns means a certain lengthening of the shaft (if the threads are 8 per inch, then half a turn means 1/16" if I'm doing my math right), which in turn means a certain tension, because the steel bolt acts like a spring--tension is proportional to how much you stretched it. Others methods are direct-tension indicators (bumpy washers), twist-off bolts, and calibrated torque wrenches. Torque-tension relationships are not the most reliable and you have to be very careful with the condition of your bolts and the calibration of your wrench.
Snug tight is what it sounds like. The Bolt Spec (RCSC specification referenced elsewhere in this thread) says, "The snug-tightened condition is the tightness that is attained with a few impacts of an impact wrench or the full effort of an ironworker using an ordinary spud wrench to bring the plies into firm contact."
That's a simple and understandable definition, but still kind of vague, given that ironworkers come in different sizes and strengths. If you don't start from a true snug-tight condition before you do your turn-of-the-nut, you won't get the full tension you need. The pre-installation verification procedure for pre-tensioned and slip-critical joints serves in part to calibrate the work crew to make sure that they're really getting the tension they're supposed to be getting, which, if the turn-of-the-nut part was done right, really means a verification that they're starting from the right notion of snug-tight.
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