I kind of assume the way they perceive that motion as like a chord strum reset except it’s like a miniature form of it. That’s kind of how it feels for me anyways for the phrasing that I learned as apart of my guitar noodling. It really depends on the phrase as likely the 2nd down is going to be a rest stroke downstroke, but this isn’t always the case.
I would also like to mention currently I can hit about 70-80% speed of the main players in this field, that 70 is if the song is just a speed demon one, but some I can take almost into the 90% speed realm. I do know when I play along that the technique I have brought to fruition is indeed the same that they use. It is relaxed, but the speed will just take time as I am almost 40 just started to learn rest stroke around 37 1/2 years of age. I may never get to their speed, but if I have a child he/she might since I have imprinted this picking gene with some phrasing as a part of my being.
And really the only thing that I upstroke string change now during a descending phrase is mainly the 3 string arpeggio run stuff (i was always naturally just fast at these style of phrases and short etude style runs), most everything else is always a half rest stroke. It has become more ingrained into my playing that doing upstrokes during a descended string change would be wrong and feel terrible to my hands. The more songs and solos I continue to learn the half rest stroke speed gets faster. Just have to devote time to your instrument if you want the speed.