What is 'Spell Stance' and how do I activate it?

I just found a weapon with an ability that says I can dodge projectiles while in spell stance, but not while casting.

I thought it might have been the magic pose that you get when you hold LT. However, the character Ghat has an ability where you fall slowly "while casting", and holding LT causes him to do this, so that must be 'casting'. I don't imagine they're the same thing.

Unfortunately I died before I was able to test it out to find out what it meant.

How do you get into spell stance?