I'm guessing you mean this 3rd party feat?
Prerequisite: Combat Expertise
Benefit: Whenever an opponent makes a failed combat maneuver check against you, you can use an attack of opportunity to attempt the same kind of maneuver upon that opponent. You must be able to reach your opponent to do so.
Attacks of Opportunity are in addition to all the normal actions you have during a round, so you could make a normal attack during your own turn in addition to using this feat.