Found my own answer by Typing in “natural” glute ham raise in the search.
From a Chad Waterbury’s (week 252)article:
When performing the exercise on the floor (natural method) the knees can’t drop down due to the floor and the upper shin is pressing against a pad or towel, therefore, the exercise requires greater force production. A good example is using the NGHR. Imagine you’re looking at someone from the side that’s performing the NGHR. If you draw a straight line from the knee to the top of the head, that’s the moment arm.
Now imagine the fella in the pic pushes his hips back and flexes his trunk forward. In that position, if you draw a straight line from the knees to the top of the head, that’s the new shorter moment arm. Therefore, trainees often push their hips back and attempt to “fold up” as they fatigue (if they aren’t strong enough) in order to shorten the moment arm, thus making the exercise easier. But many trainees can’t perform more than a few “natural” reps, so I have them use the bench until they build the necessary strength to perform the natural version.