-updates that never happen
-kick ass characters
-funny for the first 5 minutes
-Fly me to the moon
-Heart of Sword
-Peace of Mind
Anyways, the first thing revealed to me as I sought to learn the secrets of the Gatotsu was that, damn, there's a good reason that Saitou holds his sword in his left hand! Holding the blade in your left hand is pretty much unexpected by whoever you're fighting. If you charge each other, the other guy is used to striking/slashing/cutting with his right hand! Therefore, if you start running down his left side, he's going to get confused, and will have alter his strike to hit you.
I don't know how many of you play sports like tennis, squash, and badminton, but if you do, you know that a backhanded swing is harder to pull off than a forehand. So while dude you're fighting is swinging his sword 'backhanded', you have the forehand thrust, and BOOM! he's dead. The same metaphor can be used for a left-handed hitter in baseball.
Now, before everyone goes off thinking a 'stab' is weak, and can be parried easily (think Europeon fencing), hey, we all know that the katana is way stronger than a rapier. Plus, even if the gatotsu misses, it can easily be made into a sideways cut. See? The gatotsu can kill even when it misses.
So now, here's the heavy duty stuff on the gatotsu. First off, before you start practicing this technique, you should know how to properly hold your sword. First, grip the weapon near the end of the hilt, with your left hand, the edge facing outwards. Then, with your left shoulder back farther than your right shoulder, stand with your feet wider than shoulder width apart, your right foot leading. Your weight should be on your right leg and your back should be straight.
When you have decided that it is time to strike, crouch, and point your right hand at your target. Push off with your right foot, and when your right hand is sword length from your target, snap your left shoulder forwards to skewer your enemey. You should snap your shoulder forwards as your left foot hits the ground, so that you lunge at your opponenet.
Now, a big thing that a lot of people can't figure out when doing the gatotsu is the follow through. I mean, sure you've gotten the other guy, but to be truly great, you can't just suddenly stop there. You've got to go all the way. When the sword hits, take one more step forward, bringing your right arm behind you, and put on the brakes. The force due to friction should slow you down.
Should you miss, as previously described, immediately make a sideways cut, and return to a normal stance, bringing your right hand to grip your sword above your left. Defend and attack as needed in order to position yourself for another Gatotsu.
Don't be discouraged is you don't get it on the first try. Moving at a reasonable speed while keeping your arms in the proper position can difficult to say the least. Just keep practicing, and eventually, you will get it. Remember that looking cool is a big part of the Gatotsu.