When having Frozen mallet 
and guinsoo's rageblade
are both in your items, the frozen mallet passive will prog when you have 6 stacks on rageblade and you are attacking minions, small monsters and probably also champions.
Danjeng2 votesI tested it using yi in a custom. Good catch, lucky I didn't try it in a real game.
The weird thing is that I was in Highlander, but slow still applied. 6 stacks rageblade, attacked enemies.
HectonD1 votesYa I just got it too.
but it happening just when yi kills a target, then got slow, and even that is not 100% true.
I guess it will happen when the rageblade passive kill the target, then the second on hit effect goes for no where but your self
:(
HectonD1 votesNope, I just tested using jax, with frozen mallet, rageblade, proc 6 stacks, attack monsters, i didn't get slowed
HectonD1 votesIt seems happen when score a killing, for sometimes.
The Internet1 votesCan confirm, I get slowed by myself on master yi with frozen mallet and guinsoos when just autoing minions.
