Before I say anything, I'll preface it by saying that I've only recently been using sidearm shots for anything more than just getting out of trouble. I have been far too inconsistent with it to feel confident using it off the tee. Also, the more hyzer I would try to put on the disc, the less power it had. This is just a reflection of where I am in my own progress but hopefully this information can help someone else out too.
After a field session yesterday, I found that I'm starting to get a much cleaner release by simply thinking about what makes the backhand technique go and how to adapt it to sidearm. By that I mean, generating power from your legs and hips and not from your arms. I had to go back to that backhand drill where you're trying to spin your shoulders as fast as you can -- just in the other direction. It felt really strange to me, but the result was much straighter, repeatable shots. In fact, I almost have to try not to move my arm too much or too fast. It's "slow, slow, slow...POP!" With the "POP" originating in my legs. The arm is just along for the ride until the very end. I don't know why it took me so long to figure this out when I read the same idea on this site every time I visit. Muscling sidearm shots with the arm hurts and it's not very effective. I knew this, but I guess I didn't spend enough time thinking about it or working on it.
The epiphany occurred, believe it or not, in throwing putters with my index finger supporting the middle of the disc (fingers making a "V" with the middle finger on the rim". When you do this for control, you barely move your arm at all, but the accuracy and efficiency of the throw is off the charts compared to what I was doing. Then I tried standstill shots with some other grips. The run up just throws me off at this point and I start to revert back to wildly swinging my arm without any hip or shoulder again. I plan to work that in slowly.
I have no idea how far these shots are going right now, but before, 220-240 was all I could expect. I could muscle overstable stuff out to about 300, but the cost was too great in terms of wear and tear on my arm and I had no idea where the disc was going to go out of my hand. I could really feel it after 6-10 shots.
To summarize, get a feel for it with a putter, just flicking the wrist without rolling it. Try that standstill wrist flick with a couple faster discs -- don't worry about the distance, just get a feel for a nice clean release on various angles without any kind of wrist roll. When you go to add some power, rotate the torso similarly to what you do for backhand shots, try to start it in the legs and hips and don't move your arm until the instant before you release. "Slow -- slow -- POP!".
This epiphany assumes a certain level of familiarity with the various sidearm instructional videos that are out there, including the technique troubleshooting section of this website. Even if you've read it -- it's good to go back and re-read after some time. Your own experience will enhance your understanding of these articles. Old habits die hard, as they say...