When I'm shooting I like to be
1) relaxed mentally - focused on the shooting at hand.
2) attentive visually - knowing what I want to see to make a shot and being patient enough to see it without dilly-dallying around and seeing too much
3) aggressive physically - snap my eyes where they need to go, drive the gun from target to target, and really MOVE from position to position.
I don't expect to be physically relaxed while shooting (i.e. muscles relaxed). Now at other times in the match I try to make an effort to relax my muscles to ease extra tension that isn't needed. I find the mental "relaxation" is really clarity and focus on what you want to do. It "seems" relaxing to me, so I use that word. Hard to write down.