Man, what in the hell... from Joystiq:
Once summmoned, Shiva has her own commands in battle, making it very similar to Final Fantasy X's summon system. However, Square Enix has introduced a new gimmick in XIII -- the summon's Gestault mode. This is a mode which completely changes not only the appearance of the summoned creature, but the nature of gameplay in battles, as well.
Shiva transformed into a motorbike. The battle system then changed from turn-based to something more suited to an action game. Snow, riding atop Shiva, is able to maneuver around the battlefield in realtime, bashing into enemies and performing certain special skills.
So the summons are like Transformers. It's like Nomura is trying to make this game as stereotypically japanese as he can. The only thing that's missing at this point is a jpop dance number and a groping mini game. I will bet hard cash that the ultimate summon is going to be a giant robot with a light sword that the character jumps into and pilots.