These friction forces are generated by the extreme tightness of the structural bolts holding the connection together.
The "turn of the nut" method is also widely used to achieve that state of friction.
Slippage of a slip critical joint in a roof truss could result in unintended ponding effects.
Usually, this requires cleaning, descaling, roughening, and/or blasting of the faying surfaces.
Painting the faying surfaces with a class B primer also allows being in accordance with most of the design that asks for Slip-critical joint.