User blog comment:BoltOfSpades/Igetak, the Fundamentalists/@comment-8050014-20151108013629/@comment-5106485-20151108014427

The ult directly switches between them and their abilities. If you're currently controlling Ignis, you can use his abilities, and right-clicking will command him to move and attack, while Aquas can be moved around with the alt key in the meantime. Ulting causes the inverse.