TLDR: grind rpm changes extraction time and should be controlled to prevent an additional variable while dialing in a new coffee, or experimenting with other different parameters.
Longer version:
I've been using a 1zpresso J-Max grinder with 48 mm burrs for several years. Recently I started experimenting with grind rpm and angle separately to see what would happen. My curiosity was due to claims that slow feeding or single feeding with electric grinders produced a benefit, and I was curious to see what may happen with my manual grinder.
My baseline technique is to hold the grinder vertically and turn the handle relatively quickly at about 120 rpm, trying to hold rpm as consistent as possible. I rotate both hands about each other, with the grinder making small circles inside larger diameter movement of the handle.
As background I had noticed on random occasions if I slowed rpm the handle became progressively more difficult to turn, to the point where my grip on the grinder body because limiting and would slip. This seemed to be evidence a slow grind rpm was out of design spec, because it became so difficult to use without more grip or leverage on the grinder body. I expect some people with weaker hands would not be able to manage this at all.
I first tried changing grinder angle, by maintaining same technique and rpm but tilting the grinder body at about 45 degrees. When angled I could immediately tell the feed rate was reduced because grind force at same rpm was reduced. To my surprise my first puck choked up and total extraction time went from 30 to 60 seconds. I added about 10 clicks to grind coarser and bring the extraction time back around 30.
Later I tried slowing the grind rpm to about 60-80 while holding the grinder vertically and observed the same general effect as the angled grind above where a coarser grind setting was needed to avoid choking the shot and maintain extraction times around 30 seconds. However an additional challenge is it was very difficult to maintain constant rpm, as the grinder would become momentarily "stuck" requiring transient higher force to compensate and push through, leading to more irregularity in the grind rate. Ultimately I wouldn't recommend this technique as it was borderline too difficult to grind given required grip force on the grinder body, and the difficultly maintaining a consistent rpm likely contributes unnecessary variability in the resulting grounds.
Conclusions:
I'm not yet sure whether the quality of my shots is improved or reduced with either of these changes, but in both cases, changes in grinding technique had a large effect on extraction time. Maintaining consistent hand-grinding technique is an important variable affecting my shot equation that was previously unknown, to me at least. If not compensated for in other ways, such changes can at least partially explain difficulties in producing consistent shots.
For now I'm back to my baseline, but I may consider trying angled grinding again in the future.
I'm curious to know what other think and whether my findings are consistent with what others have found with hand grinders and technique. Possibly this is specific to my 1zpresso J-Max, or my specific baseline method.