Dell Precision 7920 Rack Owner's Manual page 75

Figure 27. Slide system into the rack
Installing the system into the rack (option B: Stab-In)
The static rails are a "stab-in" design. This means that the inner (chassis) rail members must first be attached to the sides of the system
and then inserted into the outer (cabinet) members installed in the rack.
1 Pull the intermediate rails out of the rack until they lock into place.
2 Release the inner rail lock by pulling forward on the white tabs and sliding the inner rail out of the intermediate rails.
Removing and installing components 75