Moreira leg push drill

Perhaps someone here (like Michael Jen f.ex.) could tell me what the following drill is supposed to accomplish, because I've looked at it and I'm not really sure what its purpose is or how it helps you keep your opponent in your guard. I may know the answer but I'm not 100% sure.

It's the drill Joe Moreira teaches both on his old Panther video series (it's on the first blue belt tape about open guard vs. standing opponents) AND in his new "Guard" book.

Your opponent is standing in your open guard and you have both feet on his hips. You push with your left and right legs alternatively on his hips. He also lets you do this as a solo drill.

How does this help keep your opponent in your guard?

I believe the drill you are referring to is suppose to teach you to use your legs to stop forward pressure. In addition, you are alternating so it teaches you not to allow your knees to come close together when you push with your legs.

Thanks, Michael.