Currently watching Taliesens interview.
Apparently, it's only slightly rng?
Basically, you press the button and then it randomly chooses a spell, and then cycles through each spell for a period of time.
(insight->glory->solace->insight-> etc.)
So you do choose the spell that you want to apply, and it works with Twins.
I dunno, it feels a bit convoluted to me.
I'm not a big fan of the whole "fish for the right buff" idea, if you miss your wanted spell, you gotta wait the whole 3-5 seconds to have another attempt at casting the right spell again.
Or god forbid you press it a quarter of a second too late/early, and you apply a useless buff to a party member that is topped/has mana/doesn't use physical or magic damage.
Seems like something that can be pretty easy to fuck up and can end up being quite frustrating.