Boost is a mechanic in the Persona Q series.
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 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 party may instead gain a Sub-Persona that has gained several levels and thus has more of its naturally learned skills available.
Persona Q2: New Cinema Labyrinth
Boost no longer affects All-Out Attacks or Sub-Personas. Instead, All-Out Attacks are performed in a fashion similar to the mainline Persona games and can be initiated if all enemies on-screen are downed, and Sub-Personas are held by rare golden Shadows. Unison Attacks can be initiated after a party member that corresponds to the attack gets a boost, in case if the opponent cannot be downed.
In exchange for the reduced focus on Boost, there are a few additional things a player can do with it. Several skills (usually physical ones) now deal increased damage when used under the effects of Boost, and the player also gains access to the Baton Pass support skill, which lets them transfer Boost between party members at the start of the turn (at the cost of 2 Party Gauges).