One word of caution - don’t rely on rest stroking on the string positionally beneath the one you’re playing as an indicator that you are properly doing USX. I spent (wasted???) some time doing this. I was playing the infamous Petrucci chromatic exercise at 185 bpm.
But guess what? Troy and @tommo critiqued me and reminded me I was using elbow to drive the motion. I knew this but for some reason I thought it was ok. I may have conflated some things I read on here about Zakk Wylde making this work for him. There are nuances we don’t need that make what he does ‘work’ and make what I was doing ‘not work’. I like to think I was sort of an outlier or interesting case, because while my motion wasn’t the most efficient, it was decently fast and the playing was clean (on my good days). The problem was that I was working very hard to make this all happen. Playing fast should feel easy.
So, it was a detour, but I spent some time doing tremolo only with a more deliberate elbow mechanic and using rest strokes the DSX way (i.e. the pick rests on the string positionally above the one I’m picking). In short order I was doing tremolos at 225 bpm. I’m not proud of this at all or bragging, because we’ve learned that even inexperienced non-guitar-players can do this The good thing that came from this is I finally felt what this ‘fast and smooth’ thing that everybody talks about it. I had a reference point. I could start applying this to wrist base DSX and forearm/wrist blend USX. I could tell right away when I wasn’t doing a motion correctly because I could compare the overall feeling to that 225 elbow tremolo.
Sorry for the TLDR nature of…probably everything I post on here. If I had more time I could make the posts shorter Bottom line - rest stroke, yes. Just be certain the motion is correct. If you aren’t sure if the motion is correct, it probably isn’t.