Very interesting question…after two months at my first dojo I was still unable to 'slap' at the end of a roll---try as I might, they were silent rolls. One night my exasperated sensei sent me to roll in the corner for an entire class. I could slap if someone threw me hard, but at all other times it escaped me. Asking 'why' I needed to practice it was, I believe, one reason they encouraged me to leave. (Word of advice---some senseis believe 'because I just told you to' to be an outstanding reason).
Now, more than two years later, I am again thinking about it.
The slap does help to dissipate energy, but it seemed to come naturally enough when needed. Before I left my first dojo I could slap when rolling by myself, but only if I threw myself into the roll, which I did for hours, looking for the secret of slapping with less energy. One day it just happened, and I'm not sure why, but think it may be that my rolls themselves changed. I think the slap times out right with the roll when I am really round in my roll. When I was newer at rolls, they were I think more oval and so my 'back' arm never hit at the right time---maybe the diagonal across my back was not oblique enough, if that makes any sense. Or maybe my back arm was too tense, so it didn't come naturally around to slap.
But keep in mind, this comes from someone who had a lot of trouble with it, also, and so I might be totally wrong. But I do have a lot of empathy for you…good luck!