I actually wondered about this a while back so i tried a build with it. I made my guy go all str and only powerstrike, I think at lvl 31 i ended with 42 str and 14 ps. I used a normal long iron mace and crushed thru shields pretty often, can't remember if I ran into some1 with a shield that i couldnt crushthru. When it came to manual blockers, I was never able to crush thru except I think 2 incidents (jumping off a ledge down onto some1 and overhead chop them).
I think dmg on a wpn for crushthru might affect the chance even thou I dont think I see ppl mentioning it when talking about crushthru. I'm gonna be looming the long iron mace 3x to test this out anyways sooo...