I've done lucario for a few months, I wouldn't judge myself as bad/good, but anyways.. "an opponent who chases you to recklessly" Yup, that would be ZSS. -.-While dashing, tap back on the control stick and Z and the same time. Your character will perform a special grabbing animation as he or she turns around and grabs.
It can also be performed by quickly dashing one way, then tapping back on the c-stick and Z at the same time for a near-instant or standing pivot grab.
Pivot grabs generally have more range than a regular grab, but are quite a bit harder to land.
Pivot grabs really aren't that essential to learning Lucario. You should only really mess with them after you've learned everything else, and even then, they're mostly just used for mindgames when dashing through someone's shield, or to punish an overaggressive opponent who chases you too recklessly.
In general, what stages would you recommend as lucario? I hate Fd, but thats pretty much it..