Cast When Stunned Tech Question

https://pobb.in/CCTzayhiBc4V

For the last few weeks of the league (praying) i wanted to try something that I've never done before. I've never run a cast when stunned build. I can see, from looking at the POB, the interaction between Valyrium, Immutable Force, etc. but where is the stun coming from? Do all hits stun if you have 0 stun threshold?

Thanks in advance.