I mean a fixing arrangement which, like SKS guards, will release if something gets stuck between your guards and the wheel/tyre - like a twig. If they don't, your wheel could jam and it doesn't take a great deal of imagination to visualise whar will then happen to you in relation to your front handlebars. I was on a ride a while ago with someone else on a surly who had cool looking metal guards which, after a short look, I worried about.
This might seem liike overfussy nitpicking but I well remember about 20 years ago, when I got back into cycling, being perturbed/p'ed off to find that when cycling along a track my front guards seemed to have detached themselves. Sod it I thought, then took a closer look and figured that my shop-fitted SKS guards had probably saved me from something worse than the minor inconvenience of popping the fittings back in.