French roll? I haven‘t heard the term before, but it is _probably_ doing what I was shown to do on day 1 of my basic course, and that is...
Roll the loose end of the strap through the spare/clear hole on the plastic buckle, if available. Then without pulling it tight, begin to roll it up. Keep rolling it until you can fit the rolled part within the remaining strap loop.
Hard to describe, easier to demonstrate.
Another way of looking at it is rolling the strap in on itself. You can actually get them pretty tight, and if it needs to stay that way, I have seen people use gun tape to tape it up.
One negative is it can take a while to do, so I usually haven‘t bothered on the ruck straps that hold my valise to the ruck, or on the main compartment straps. I usuaully try and tuck them away somehow. Maybe I am supposed to be getting yelled at for that and have been lucky so far, though.