You are right on with the muscles. And, you should never "flinch" anything.
If you are using a caliper style release, you slowly pull back on the trigger as you are applying the back tension. Your release hand needs to be relaxed.
With a BT release, the release is rotated as your back muscles are drawn together. Thus, firing the release.
Regardless of what release you use, the firing of the release should be somewhat of a surprise.
I hope that helps some.
Take a kid with you. Show them the Great Outdoors. Pass it on.