I tried to do the same with other body parts, like flexing my arm and holding it back with my hand as I tried to straighten it, but the force from releasing that doesn't seem to be more powerful than simply punching out the arm flexed to extended without holding it back in any way.
The answer above is spot on. But let me respond (in kind, i.e. like the answer above) to this aspect of your question, hoping you can relate it to the answer above.
The arm question is exactly like the finger question; you're just looking at it kind of in the wrong way.
If you bend your elbow, say, to 90° at a distance of 1 inch from a pane of glass, then try to extend your arm as forcefully as you can with someone holding your arm back at the wrist, if they suddenly let go, your arm may well have enough force built up to break the glass. (This depends somewhat on the strength of your triceps, not an overly strong muscle in non-athletes.)
However, bend your elbow, say, to 90° at a distance of 1 inch from a pane of glass, then extend your arm as forcefully as you can into the glass, It probably won't break, because you can't build up enough force in your triceps over the course of that inch without resistance.
This is exactly analogous to the finger example explained above, except for the size and appearance of the fingers. It is analogous even to the extent that we do not use our finger extensors against resistance very often, much like our triceps.
You can apply this to any muscle-joint group in the body. The outcome will be roughly the same.
I do hope that helps explain the answer to your question.