Here's the short version of IronMan's X-Rep definition (You'll have to check out their ad for the bogus
different lighting, different pose, dieted down, tanned and oiled before and after shots).
1) Peform a regular set to positive failure.
2)Move the bar, dumbells, machine arm to a position where the target muscles are in a slightly stretched postiion.
3)Do 4 to 6 partial range reps in this position to failure.
Or, like a said above, "burns".