When the supreme court reviewed whether the religious freedom restoration act was legal under the constitution, which power was the court using?

One of the Supreme Court's major powers is the power to review laws to determine their legality under the Constitution. This power is called judicial review. This an increasingly controversial power, as it becomes more often used. Their is a great deal of subjectivity involved in this process, and since the Supreme Court consists of only 9 justices, it gives a small, unelected group the ability to strike down virtually any law.

Many argue that this gives the judicial branch power beyond its intended scope, especially with regard the legislative branch. Indirectly, this also gives the executive branch a great deal of power, since the President has the power to nominate new justices.