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.
I 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.
Nope, I just tested using jax, with frozen mallet, rageblade, proc 6 stacks, attack monsters, i didn't get slowed
Can confirm, I get slowed by myself on master yi with frozen mallet and guinsoos when just autoing minions.
