I'm guessing that it only works on thrust weapons like spears but will it also work on the rapier. And it says strengthening counters so is that just for parry and repraisal? What all weapons would it work with?
A counter happens when you hit an enemy in the middle of certain actions, namely their attacks, but it also happens when they recoil off shields and many other actions. It does extra damage, which the Leo Ring boosts. Best used with spears since they let you poke mid-attack with little risk to yourself. You can tell you've countered as a little spark will appear at the point of impact.
I rocked it with the DSS for a while, cosplaying Ornstein. Didn't notice any huge improvement but then I'm way to scrub to count up my damage in real-time and stuff. Did kill people pretty good though.