This was answered 2 posts before yours. Search and read before posting.
I think he is asking for another reason and I always wondered the same:
Say you have enough focus for AS and signature shot, is it worth it to delay the signature shot by 1 GCD to apply HM with AS via the glyph to get increased damage on the signature ability right away?
Or is the HM applied and taken into account when you simply use the signature ability anyways? (like hit CS button, HM gets applied, HM improved CS hits target)