Hypothermia gem question - When is it counted?

If I put Discharge in my helm that has "socketed gems have level 20 Hypothermia" and let's say I ONLY hit the boss once, does that hypothermia get added to the hit?

Or is it only for follow up hits after you've already chilled the enemy?

Enemy must be chilled for the first hit to be implemented.
Then only if you use Skitterbots !?

Yep, as above, need to be chilled when hit, not on the hit that is applying the chill.