Before attempting a difficult task, the caster takes a moment to receive a magical boost to their self-confidence. This mystic pep talk gives the caster a 15% bonus to one die roll (caster's choice) needed to complete the task. The bonus may be used to affect a POW check, attack roll, skill check, etc. For example, if the caster were about to cross a narrow log high above a chasm, they could invoke this spell and gain a +3 bonus to their Acrobatic skill check.
Once cast, the spell may be maintained until needed. Once the bonus is used the spell would need to be cast anew to gain another bonus.