When we use the term (stepping) under himself. It means that the horse is moving from behind on the bit in proper carriage. There are a couple of new posts on here that are talking about this. You might want to take a look at them. One has photos and it may help you understand the concept and what it looks like.
Thanks. (OT do you think im too big for him? Since that pic I've lost like 20 lbs) he is my buddy. The bit I had him in was terrible. I got in in something new and he is better. Do I just push him into the bit? And get him on the vertical? Will he do it if I lunge him? If I take a vid. Of us will that help? I ride him Bareback so I don't know if that helps.