I havent confirmed this, but i am fairly certain that amumu's tantrum reduction on hit (-1 second) does not currently apply on DOTs (like teemos) or non-primary attacks of several kinds (abilities, AOEs, after affects [cho-gath's vorpal spikes])

again, I HAVE NOT CONFIRMED THIS, but am reporting what i believe i saw in a 40 minute match. i will try to get confirmation of this later tonight, but figured i should let you know asap

It only works on physical attacks if i am not completly mistaken....would be nasty vs katarinas ultimate.