Persona Q: Shadow of the Labyrinth
Boost is a mechanic similar to Smirk in Shin Megami Tensei IV. Characters enter the Boost state whenever they score a critical hit or exploit an enemy's weakness. Boost ends immediately if the character is hit by an enemy attack (even if said attack got absorbed) or after one action, however it can be maintained by exploiting weaknesses continually. Boosted characters have their attack priority increased, meaning they move sooner during the turn, and the cost of all skills is reduced to 0; additionally the more characters Boosted during a turn, the greater the chance an All-Out Attack will occur at the end of the turn. Earning Boost may randomly cause your party members to initiate Co-Op Attacks, however this is much rarer.
At the end of the battle, there is a 10% chance of getting a Sub-Persona, which increases by a further 10% for each party member Boosted at the end of the battle. Rarely, if the party has been Boosted very frequently, the player may instead gain a Sub-Persona that has gained several levels and thus has more of its naturally learnt skills available.