When learning to do a forward roll, many instructors have a kneeling form that they say is lower-impact and easier than a standing roll. Well, the way that I've seen it done most often is not really a kneeling roll and it is not even easier than a standing roll. This is because typically it is done by beginning in a kneeling position with the front knee up - and then standing up into a crouch. This is actually counterproductive. Check out how we teach this first kneeling forward roll exercise.
(AAARGH! that infernal Google embedded player makes me crazy. It worked for a while then puked out. The above is the link to the video.)
Things to notice include: the front knee and shoulder start on the ground. The hips rise but the shoulder does not come off the ground - so this is truly a no-impact exercise. Not only is it no-impact but it is exceptionally low-speed. The benefit of this is that it really improves the muscular coordination of the torso and hips better than throwing yourself through the roll.
Now, for pure entertainment value, check out this forward roll...