I remember a couple of people asking about this mastery recently but I can't find the thread now, and the other stuff I found online about it is pretty misleading. Anyway, I had maxed out collar tech a while ago, then stopped using it, but I was playing around with my masteries today and was able to test it out and show that it does work.
So this is the same fight with identical circumstances. Same Sentinel vs. same Juggs; both bars are after same parry stun MLLLM combo. First without collar tech, second with 5/5 collar tech.
And for what it's worth, I'm still not going to use it, I don't think it's worth it for me.