This argument comes up a lot in grappling when coaches will say 'don't use strength'. Well, that's obviously bullshit. you have to use strength to get anything done. What you shouldn't do is use stupid strength. Which is to say, you don't get much out of bagwork by going in and just throwing as hard as you can with no attention to technique. In general you should be throwing nearly as hard as you can while maintaining good technique. I do a lot of technical work at lower power, but I always have rounds where I throw hard because as you say it's important to learn to do and develop the kinetic chain and you can't do that just throwing light. Work on your technique, then go throw hard and pay attention to where you're falling apart technically when you up the power, work on that a little lighter, then go back to throwing hard. Do that over and over until you can throw hard with good technique. The key here is that whatever you're doing you have to be thoughtful about your goal. If you're throwing light, you're only doing it to groove good technique before you go back to throwing hard. If you're throwing hard, you're not doing it just to beat up the bag, you're trying to develop power while using the proper motion so you should be self monitoring your technique the whole time. One of the things that drives me crazy as a BJJ coach is people not paying attention to what they're doing on the mat. Why are you there if you're not actively trying to get better? I find bagwork very mentally draining because I try to make sure I'm paying close attention to what I'm doing on every strike and that gets hard as you get tired, but otherwise what's the damn point?