Great view of Sierra’s picking hand here, incredible DBX technique!
One takeaway for me was the size of her pickstrokes, it’s inspired me to consciously try and use larger pickstrokes when working on my own DBX, feel like I’ve had some success with it tonight. Anyone else noticed this in their DBX playing or am I reading too much into this? 
Great example. Where she really starts to burn is in the fiddle-tune-sounding Mixolydian thing at the end (not sure what it is, I assume a traditional). Like most of her generation of bluegrass mando players, her entire style is heavily influenced by Chris Thile (she’s said it herself), but while Chris usually transitions from DBX to a mix of USX and DSX to get his trademark machine-gun maximum speed lines on non-1NPS parts like here, this clip shows she can get pretty darn fast and stay more or less pure double-escaped.
The tune is Big Mon, it’s a fiddle tune that is commonly played really fast! I love this version of it by Jake and Trey 
Yeah it’s interesting, very similar in that they do the DBX thing and (I’m assuming) the faster optimised DBX thing where it becomes more USX/DSX oriented depending on the phrase and then they will both do the typical mandolin elbow technique for super fast single string stuff but Chris is DT and Sierra is RDT!
I’ve seen Tony Rice occasionally break out elbow technique for super fast stuff as well, normally he’s a finger/wrist DBX player like Kiko Loureiro and Martin Miller (I think):
Ah, yeah, I am theoretically a roots musician and I’m feeling rather bluegrass illiterate for not knowing that off the top of my head 
. It certainly sounded familiar. So many fiddle tunes…
Great version by Jake and Trey.
That Tony Rice clip is a great find too, I’m not sure I’ve ever seen him break so clearly into shred territory anywhere else. He mostly lived at the high end of DBX speeds, like the whole world of modern flatpickers that he inspired (though he was not nearly as strict as most of them about alternate picking and even claimed he had no particular system about pick direction, though I doubt that was the case, he just wasn’t conscious of it).
As far as I’ve seen Chris Thile does often still use wrist motion even at the very fastest speeds, it’s just the more economical single-escaped version unless he’s forced to stay DBX for 1NPS lines. An example of the latter is here, where he plays the 1NPS double-time arpeggios in the middle of Bach’s D Minor Chaconne for Violin, one of the fastest passages in any well-known piece of music—in his defense it’s an early work-out performance, but you can hear him struggling to control the dynamics and even hitting neighboring strings because he can’t use the single-escaped motions he favors at those speeds:
That’s just a side note, unrelated to his single-escaped mechanics.
For sure he will often break into elbow motion or some mix for very high speed lines as well, but I don’t know if there’s any rhyme or reason to when it happens (or if even he knows about it!). A lot of players advocate against elbow motion because sounds too stiff (Al Di Meola says that) or is hard to control (Guthrie Govan says that) and except in edge cases like the above video, Chris can hardly be accused of either of those! I think partly that’s because his elbow motion is so precise, and partly because he also has excellent wrist motions and can mix them in freely to serve the line.