Amumu outregens Ashe autos on Twisted Treeline with no keystone or items [GAMEPLAY]

So I was playing a Twisted Treeline and there was an enemy Amumu on the other team that seemed to be doing some pretty wild regenerating. At one point he was standing next to our Ashe, she was autoing him and he got to 105 hp, and just stopped taking damage completely, and started healing. It may have something to do with the back animation? Attached is a dropbox file of a .rofl, I'm not technologically adept enough to convert it to a video, sorry.