A repealer is a clause in a bill that sets the date that the law will expire or cease to exist. A reverse repealer sets the repeal date as the day before the law is to take effect. For example, if the law were to be in force from and after July 1, 2014, the reverse repealer would have the bill lapse the day before, on June 30, 2014, so that the bill would never become law so long as the reverse repealer is in place.
A reverse repealer is inserted in a bill when the bill needs more work. It ensures that the bill cannot become law as is and that the bill will go to a conference committee for further revisions.