It's a penetration+1 check, from the debug log:
spec_mr pen 12 mr 1
A H3 Smite in the same magic-neutral province shows up in the debug log as:
spec_mr pen 11 mr 1
I agree it's not spectacular as an anti-SC device, but if it's an extended battle between two combat SCs, once it triggers, there goes your defense and attack. I have experienced this pleasure first-hand from the wrong side of the shield.
