Actually incentives earned are added to a team's salary cap at the end of the season. See the relevant section of the CBA below:
At the end of a season, if performance bonuses actually earned resulted in a Team’s paying Salary in excess of the Salary Cap, then the amount by which the Team exceeded the Salary Cap as a result of such actually paid performance bonuses shall be charged to the Team’s Team Salary for the next League Year (i.e., reducing the Club’s Room in the next League Year). For the purposes of this Subsection, the Team’s Room at the end of the season shall reflect any charge to its Team Salary in the current League Year resulting from its election to Carry Over Room to the next League Year pursuant to Article 13, Section 6(b)(v).
It's possible for a team to carry over negative cap space into next year because of it though.