The demo -- which, we reiterate, was very short -- was taken from an early part of the game. Motomu Toriyama, who was controlling the demo, took control of Snow in order to show us his partnered summon, Shiva. Each character has a partnered summon and it seems this is story specific, rather than a Final Fantasy VIII style "Junction" system.
Toriyama quickly initiated a battle and explained how the new "assault" combat option automatically selects the most approprate combat option for you. In other words, it's an autopilot for the smaller scale combat situations you're likely to come into contact with while grinding -- and if we know Final Fantasy, there'll be plenty of that.
Soon Toriyama started summoning Shiva. It's a similarly ridiculous animation to Lightning's Odin summon which you saw in the latest trailer. She emerges from a giant bubble, covered in Final Fantasy X style glyphs, which then freezes and breaks apart. Shiva then takes place by Snow's side as Lightning and Oerba disappear from the battle.
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 Gestalt 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. The mode ended fairly quickly, so we didn't get a chance to see it in detail. Something tells us we would've been shown more, but we'd already been waiting around 20 minutes for the demo code to arrive -- not to mention the obligatory pre-demo PS3 firmware update -- so we were running low on time.
The Gestalt mode should give the battles in Final Fantasy XIII an extra spark. It's probably not enough to convert action fans to this RPG, but it should be a fun, new addition to the series. No doubt Square Enix will be teasing us fairly substantially with new summon animations and Gestalt modes.
We asked what form Odin's Gestalt mode would take, but Kitase refused to discuss any summons other than Shiva. Though, he did say that fans of the series would probably be able to figure Odin's transformation out for themselves. Mountaintop target-shooting lightbolt mini-game, it is then.