I define "on the rivet" for myself as going more than 150%. So, it ain't going to be for long. Maybe 2 minutes, then take a dirt nap beside the road. That's pretty much a prolonged sprint. You're recruiting what will make the power for that long, and hopefully staying low.
Watch a crit on youtube, you'll see a good amount of "on the rivet" footage. You know when you see it, and if the person does a nice power overlay.....you'll see the watts they're doing.