I just leveled-up my gray knight's magic ability to...I think it's the fourth tier. Anyway, I got a message saying "New magic unlocked! Press X, X, Y." I could understand that being a new combo unlocked when leveling up, but this...hm. I've long-since unlocked the X,X,Y combo attack; I just don't get this thing. If it'll help to know where I'm at, here's what I've got for magic now:
CANNON BALL: RT+B
HAILING ARROWS: RT+Y
DROP CANNON BALL FROM AIR: RT+Y (in air)
From there, all I can say is, "pbbbbbbbbbbbbbbbbbbbbt."
What you probably saw was the magic combo unlocked (X, X, X, Y).
When your magic is high enough and your level is high enough, you can do a magic combo (X, X, X, Y) where the heavy hit adds a magic effect (in this case I think it's a guaranteed critical). You can add an extra Y hit where you dive forward afterwards when you learn the X, X, X, Y, Y combo.
The magic you can use via the RT is + Y (splash, midair changes to diagonal for most), + B (forward projectile both on ground and midair) and + A (magic jump).