For reaching legs and downwind legs, it is preferable to have the strap as tight as possible or at least "fairly tight" to get better connection with the boat and allow yourself to come in easily, also keeping you out of the water/higher up to prevent contacting waves. On reaches, you can be hiking, but sailing the boat extremely flat, more than upwind, so this is important. Upwind, a looser strap allows you to get more outboard and thus better leverage against the sail forces, if you have the fitness to maintain a position there (very demanding). So this method is mostly for people who prefer a looser strap upwind but would like to tighten it for other legs. If you already sail with a reasonably tight strap, it's probably ok as it is. It's certainly worth experimentation.

We have not tried it on a topper but I don't see why not

Fad Skates Might be. I'm not entirely sure though as I don't own a topper myself but from what I've seen the attachment to the back should be about the same.