Whether experienced or not everybody operates the same way when recurving a door - trial and error, but not so much on the errorI have seen/read the methods. I just don't trust my skill level to pull it off correctly on first try. I'd probably break the window....
And no, I have no strap.
Pressure is applied with the chosen method, door bends and the thought "that's gotta be enough" enters the picture. Pressure is removed, and nope, not enough curve so repeat. It's really pretty straightforward.
I've recurved doors with windows, didn't loosen them or anything, and haven't broken one. I don't know anybody that did break one, but would appreciate hearing the story of how it happened from someone who did.