You imbue the target creature with a special combat sense that grants a +2 competence bonus to its base attack bonus (maximum BAB +20), possibly granting extra attacks while using the full attack action. If this spell would raise the target’s base attack bonus to 21 or more, it receives a +2 competence bonus on attack rolls, but gains no benefit for having a base attack bonus higher than +20. You don’t possess this additional base attack bonus for the purpose of prerequisites.