Orb of Slow ability, simple question

Well, I've been messing around with WE, trying to trigger some spells etc etc and I got a question about one of item abilities.

Ability known as "Orb of Slow" (the one which has % chance to use preset ability while attacking) has interesting effect, may be used for many purposes, however I need to know only one thing:

Does it trigger in the moment I deal damage to enemy OR minimal time before/after the unit gets damaged?

I am sorry if I posted in wrong section or if this question was already asked, I couldn't find it.

I would assume on damage. Seems silly for it to trigger at any other time.

Why would it matter when exactly it triggers? Unless you want to detect the buff in a trigger that runs on the damage event, in which case you could use a simpler ability like Slow Poison.

That's what I exactly wanted to do.

Thanks for the advice!

As an aside, if you ever want to make an autocast attack-based ability this way, use Cold Arrows. All others attack-based abilities fail in my experience: Frost Arrows can't use custom buffs if I remember correctly, Black Arrow is hardcoded to not affect heroes, Searing Arrows doesn't use a buff at all and Incinerate, well, it wasn't around when I did my tests so it might work but it has a whole bunch of other issues so it's less hassle to just use Cold Arrows instead.