When you open your hands, your wrist muscles flex more, making your wrist slightly larger than with a closed fist (try it). If you want a little more space in your handcuffs, you open up your hands as big as possible, so then when you relax into a more fist-like position you have some wiggle room (or enough room to slip out of the handcuffs if that’s your plan). I would guess most cops know this.

Edit: I’m not saying this is definitive, but I think it passes Occam's Razor when it comes to the open hands.

When you open your hands, your wrist muscles flex more, making your wrist slightly larger than with a closed fist (try it). If you want a little more space in your handcuffs, you open up your hands as big as possible, so then when you relax into a more fist-like position you have some wiggle room (or enough room to slip out of the handcuffs if that’s your plan). I would guess most cops know this.