The power sub does not include a heavy thrust bearing to carry the hook load, or a high pressure wash pipe & packing seal arrangement to conduct drilling fluids while rotating. To provide these functions, the power sub must be screwed into the bottom of an oilfield rotary swivel. The load-carrying rating of the power sub is limited by the swivel it is suspended from.

So, a power swivel is the combination of a power sub and a rotary swivel into one short assembly. The result is a hydraulically-powered rotary swivel called a power swivel.