So I've trained BH loops before, but it always felt wrong. After spending the past few months gradually improving my close to the table BH counters, I decided to try it again, this time integrating what I learned closer to the table and morphing it into a looping stroke. I figured that it was possible because my training on the FH side revealed that all FH strokes from blocks to power loops, against all spins, are really just variations of one stroke. After a little over a week of trying different things, I think I've finally found the right technique!
Throughout my journey to pursue better technique I've found it just crazy how wrong those top level players/coaches on YT are. They all learned their craft as a little kids and have no idea why their technique is so good. They often focus on specific things that give them the edge, without realizing all the countless hours needed to shape all other aspects of their technique to make those little things even matter.
Look at this video for example, where a ~2500 level popular coach/Youtuber is analyzing Darko Jorgic's BH. It's clear to anyone with effing eyes that Darko's elbow moves a ton in the video, even in the frontal view, yet he keeps stressing how the key is his elbow is fixed, that it's "non-negotiable".
In the below FZD BH video you can see the same thing. He has one of the more compact BH motions and it's hard to see the elbow movement from the frontal view, but go to 27 seconds in on the side view, it's clear that he moves his elbow forward a good amount.
After a lot of trials involving various videos' teachings as well as paying attention to what people actually do, I found that the BH loop shares some key similarities with the FH loop. One such similarity is in the backswing, specifically that it's mostly passive, and that the completion of the backswing occurs at the beginning of the forward swing. At maximal backswing the forearm is level or behind the elbow, and the wrist is cocked back, but it's not accomplished by bending the forearm and wrist back. Instead, the forearm achieves its maximal backward bend by keeping the elbow joint relaxed while moving the elbow forward, and the wrist achieves its maximal backward bend by keeping the wrist relaxed while the elbow and then forearm move forward. Then as you approach the ball the elbow finally becomes fixed, which transfer energy to the forearm, and lastly right before contact the forearm slows down to transfer energy to the hand/racket (i.e. the wrist snap).
This whole sequence may be shorter or longer depending on the incoming ball, but you must execute all of it to achieve the highest quality possible for each shot. The forward movement of the elbow in the BH stroke is important not just in power generation, it's also very important in timing. Like on the FH side, the forearm snap can be a bit earlier or later while the elbow moves forward to adjust for any misjudgments in timing. That may not be super useful when you're very close to the table, but take a couple steps back and try to loop and you'll have tremendous problems with power generation due to missed timing, if not missing the table altogether.