The game was called Gogetsuji Legends outside Japan, lacking the Power Instinct title, strangely enough. This would be a trend for the series in its subsequent western releases, officially phasing out the Power Instinct branding (although is still used informally by the fan community).
The battles are in teams of two characters, a leader and a partner. Super Kurara and Pochi are individual characters; because of this, Kurara and Kinta gained new moves. Otane is also made playable, bringing the cast to 16 characters. A Super Block and a Charge Attack were brought into the gameplay. A Super Block can nullify the opponent's blow and allow for a counter attack, and a Charge Attack can immediately knock down the opponent or launch them into the air for another attack. There is one new playable character, (Kuroko), and a new boss, (Chuck). All of the characters were given new special attacks. However, the majority of them were exclusive only to this game.
Source: Wikipedia, "Power Instinct", available under the CC-BY-SA License.