This topic comes up from time to time.
I think this figure is especially true when grinding out mechanics. I'd say I make the most progress in changing fundamentals throwing at 50-80% effort. On the low end when it's really confusing to me or my body. On the high end when I'm moving more safely and trying to grind the mechanics and make it consistent for on course use.
The meaning of that effort seems to subjectively change over time. Just this week I'm gearing up for bigger pumps and movements to force my form to change and that means that I'm putting in more effort. But when I'm warmed up, the 80% I throw at now feels much smoother and less taxing on my body than any form before, as is pushing past that. Then at some point the form starts to break, and it's at that margin that more and different progress can occur over time.
I think when the mechanics are becoming safer and more reliable you need to start pushing higher than that 80% to improve your maximum distance potential and understand your rest and recovery cycles. Then that push might reveal new mechanical problems and you either back off a little to fix it or keep reinforcing the poorer mechanic. Most people seem to get caught in some trap in between the two to some extent.
