It's not the best build: it's just something new for people who get bored doing Philo/Hog.
It turns Amumu into a ganking machine because his Q scales off of 100% AP and people finally stopped maxing Q last and started maxing it first.
If you go
Armor Yellows/Reds and AP Quints/Blues you can turn Amumu into a sickening gank master. But if you don't get the ganks off, you set yourself back pretty hard because you're not maxing out the E and getting free damage reduction.
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Realo won gold at the Sex Olympics with a BROKEN FRIGGIN NECK.-Voltch