Saw a couple posts on other threads.  There is really only one way to handle slack lines.  Hold your arm straight over your head and bend as much as you can to try and get the lines tight.  Do Not run, particularly at a contest.  You may save the model just in time to fly it into somebody's head! At any well run contest you will be disqualified from the flight, and maybe the contest, if you run outside the pilot's circle to save a plane(safety reasons).

When and if the plane hits the ends of the lines having your arm extended will give you about 3-4 ft of movement to soak up the shock so the lines won't break.  On any high performance plane, from a Fast Combat to a Pro stunter if you hold the handle solidly and in close you can easily hold it tight enough that the lines will break when the plane hits the end of the lines.  With your arm up high nobody( I don't think) can hold the handle solidly enough to break the lines.

Never, never, ever grab slack lines with your hand.  The lines can easily cut right through a finger or two at worst, and at best can give a very dirty, nasty cut.  The emergency room bill alone will more than cover a brand new engine, plane, and a down payment on a new car!