Does Liandry's Torment proc when OTHER champions slow the enemy?

(cross-posting from http://www.reddit.com/r/leagueoflegends/comments/2mqvzl/does_liandrys_torment_proc_when_other_champions/)

So I'm referring to this passive:

UNIQUE PASSIVE: Dealing spell damage applies a damage-over-time effect that deals bonus magic damage equal to 2% of the target's current health per second for 3 seconds. This bonus damage is doubled against movement-impaired units and capped at 100 damage per second vs. monsters.

I know the movement impair is useful when YOU have abilities that stun/slow/etc. But does it proc if someone else is doing the CC? For instance, if I have a Liandry's, and my teammate Amumu stuns the enemy, and I do non-CC AP damage to the enemy, does the Liandry's double damage proc?