Different Method of Dunking - gotta be illegal but I can't find how!

[drinkeii]

Three kids in my IM league have come up with this little setup to dunk a ball - the little 100 pound guy runs toward the hoop, while the other two set up, one bent over completely, and the other one leaning a little forwards with his back against the second guy. The little guy runs toward them, dribbling, and then runs up the back of the second and third guys, then dunks. Only one step on the back of each kid, and he dribbles on the way up. I know this is illegal, but darned if I can find what to tell them is wrong with it. I assume there is something about assisting the height of a player, such as picking a teammate up to help him reach the basket, but I coulnd't find that either. They haven't tried this in a game, but I want to be able to tell them why they can't do it before they do. The kid isn't traveling, technically, because he is establishing his pivot foot on the back of the second kid, and then his other foot is going on the back of the third. He then goes up and dunks.

I know this sounds silly... when you're done laughing, please let me know what you think! And thanks in advance for the help.