Can only be learned through years of entirely free training under the fly-catcher in New Pittsburgh. Opens the door to 3 shiny medals. As the game itself says, "perfect timing really makes a difference!" It is generally accepted that a perfectly timed strike doubles your attack and defence for one round.
Timed fighting is a cheap way to reduce server load and generally slows your attacks down by at 4 to 7 seconds. It also repeats on a loop, handy for reading monster descriptions. To use the multiplier skip one round count and hit on the next e.g. if the first attack is in 4 seconds, you can strike at 4, 8, 12, 20, etc.
Timed Fighting also works very well with Automatic fighting, with one click unleashing a flurry of death.
Defeat that pesky fishing Joker 186 times, though, and the payoff will have your head bobbing in the rhythm of a metronome in no time! Here's one tip: http://www.metronomeonline.com or, if you need a visual only metronome that doesn't make noise so that you can play during class http://38i.biz/metronome/