Its called Emergency Reserve Thrust. On each minor model of the 737NG you will get the next highest certified thrust rating for that model/engine combination if you firewall the levers. So on a 737-800 with a 24k plug you get 26k. There are altitude and airspeed restrictions to limit this to the takeoff/go-around regime. It should be noted that if there is no higher certified rating for that model/eng combination you do not get any extra. So on a 737-600 with 22k plugs thats all you are going to get. A little something extra from Mr. Boeing to take care of Mom and kids.