%chance of X procs when?

I still don’t understand when some of these interesting-looking proc chances can proc. For example, I recently found a Mythical Albrecht’s Duality, which gives a “5% chance of 51% CDR”. These and other skills which do not stipulate the conditions of proc (e.g. it does not say “x% chance on attack” or “on block” or such) base their % chance on what? 5% chance every second / full moon?

thanks

On casting the spell.

Thanks. Still not 100% understanding. So in that case it’s 5% chance of 51% cooldown reduction of that one spell (and not all active spells currently on cooldown) on casting that one spell?

If a skill has 10 seconds cooldown, and you use it, you have 5% chance to remove 5.1 seconds of the cooldown from the skill, therefore resulting in a 4,9 seconds cooldown instead of 10.