I’ll defer to a video @Troy made on categories of motion mechanics (no idea if this is being superceded as part of the revisions to the pickslanting primer). I think the latest from CTC is acknowledging that the categories of motions are often blended together in different ways.
To me personally, what I think of as an “elbow mechanic” is a movement dominated by flexion and extension at the elbow joint, typically with little or no “forearm rotation” component. It’s in the vein of something like Vinnie Moore or Rusty Cooley’s tremolo-style picking where the forearm and wrist are very rigid and everything below the elbow is oscillating back and forth in line with the flexion and extension of the elbow joint.
When I think “forearm rotation”, I’m talking about any movement where the main driver is rotational motion around an axis that runs parallel to the long bones in the forearm. There may be examples of this where some movement at the elbow is visible, but it’s more a side effect of the rotational movement, or stabilizing/antagonizing the rotational movement.
But for the sake of the “tremolo endurance” discussion, the key to me is not the contrast between an elbow mechanic and a forearm rotation mechanic, but the contrast between either of them and a wrist mechanic. (and yes, there is a small wrist component that’s an ingredient to “forearm rotation”, but it’s secondary to the main driver of the motion, which is rotation of the forearm).
I’d say there can also be times where someone like Ihsahn is doing a mostly “elbow mechanic” but has a minor “forearm rotation” component added in.
Edit: I think there have been a few times in the past where I’ve engaged @Troy in gazing into the abyss over the categorization of movements that blend forearm rotation with wrist flexion/extension. In particular, I think I have a tendency to give more weight to the role of forearm rotation, even when the amount of visible rotation is quite small. There’s probably also some fuzziness around the role of movement around the carpometacarpal joint of the thumb in that blend, especially when the forearm-rotation component is through a fairly small range of motion. Overall, I think being aware of the various potential ingredients is more important than setting hard lines about how to categorize examples that are blends.
Link to @Troy’s “4 categories” video:
Edit: I originally linked the wrong video, here’s the right one: