Having been a mage for many years now i have to agree with Bluehazee here.

If you take the Immo staff for example. Fireball and Timeshift or any type of DOT damage makes it proc more often.

Try it yourself. Clear a Maus map solo with a mage just using the Immo and no skills and then try the same using fireball before u spam Immo. U will see the difference in proc rate straight away