I’d imagine that RDT variations (radial flexion and ulnar extension) are very common among wrist and wrist-forearm players. Typically the paths are more shallow (8-0-2 on the clockface), rather than the very steep path @Troy has demonstrated. The shallower forms aren’t as fast, but they can come from an array of common grips and setups, and they’re capable of producing any escape.
The steep, hyper-fast RDT alternate picking form that @Troy has demonstrated is a DSX movement, and requires a very particular setup. The three finger grip and highly supinated setup are quite unusual. It could even be considered “wrong” by a lot of instructors.
As for Shawn Lane’s USX DT movement, this is my take. I’ve spent a lot of time watching footage of Shawn and analysing his mechanics. I can perform this movement, and I’m very aware of it’s strengths and it’s weaknesses.
Shawn’s playing is difficult to analyze. Most of the footage of his playing is low quality. The only high quality video available of his playing for the longest time was the Paris DVD footage, and there really aren’t many good angles of his picking during that performance. His technique had declined somewhat by that point, and his playing became more legato based later in his life. Shawn almost always wore long sleeves to cover his psoriasis, and his weight can make it difficult to analyze his movements.
The DT movement that Shawn used is almost a unicorn. Many players will find the trailing edge grip that Shawn used to be uncomfortable, and with a leading edge grip the mechanic can be prone to garage spiking unless you dial in just the right amount of flop in the grip, and that can make the pick prone to falling out of the hand. With the trailing edge grip, the movement really does require a reasonably small, pointed pick like a Jazz III (which Shawn used). Using a rounded pick often results in the movement vibrating the rounded pick uselessly back and forth on the string. Shawn himself said that he needed the point of the Jazz III.
As with many other notable players, Shawn really didn’t have an explicit understanding of his picking movements and some of what he says when describing his picking isn’t accurate, and it can be misleading. You should absolutely listen to what he said, but pay more attention to what he actually did.
Moreoever, Shawn’s technique is more than just his DT picking movement. That movement is coupled with highly efficient fretting hand mechanics and a system of line construction which facilitates playing lines that can utilize the speed of his DT picking movement. Shawn also made extensive use of legato (including hammers from nowhere) and hybrid picking (middle finger) to facilitate the parts of his vocabularly which don’t fit with USX.
Without the line construction system to build vocabulary to utilize the full capability movement, the fretting hand mechanics required to play that vocabulary and the facilitations to cover the limitations of the movement, the DT movement is “just” a fast tremolo picking movement.
I think the limitations of the movement would actually be quite surprising to some people. There things that the movement doesn’t do particularly well (or at all). For example, the form really doesn’t facilitate smooth sweeping across several strings. Shawn actually demonstrates some sweeps on Power Licks, and he doesn’t do it particularly well.
There’s a lot which I doubt Shawn could have played with his mechanics. Some which would actually be very natural with other mechanics. He just didn’t play things he couldn’t play.
Also, I’m quite convinced that Shawn’s absolute fastest movement has a very significant elbow component. Watch the form change at the 37 minute mark:
Notice the much greater sleeve movement when the “warp” kicks in. Notice the shoulder movement. Notice the movement in the pectoral region.
The DT muscles have secondary function in elbow movement, so these motions are natural synergists. My current belief is that at the fastest speed, the primary picking cycle is primary elbow, but with the DT movement provide a little extra “whip” for accents (and faciilating chunking and synchronisation) and a little more range of motion for string changes. I started experimenting with this idea just last week and I was able to perform that movement pattern within a couple of days.
