I don't think that's a right answer. If the substitution is just a high speed swap, then why even swap? Just dodge and that's it.
I think the substitution is a couple jutsus in one. A swap jutsu, where you switch places with a nearby object. And a transformation jutsu, where you transform the said object into you, and possibly vise-versa. This is supported by the fact that people say, "A substitution!", as if it was an object, not an action. What is really confusing is the earth substitution that the Third Hokage did. Where on earth did he get all that mud?!
But let's not go and try to apply real-life physics to this. Remember, this is the series that has 2D ink paintings become 3D life-like animals.
EDIT: Substitition is handy for distracting an opponent and/or escaping a trap. But another thing that's confusing is the hand-seals. We see Sakura doing like half a dozen hand seals, but earlier in the series when Kakashi is being grappled by Naruto, Kakashi does the substitition without any seals at all.
Last edited by Numbnut10; 05-04-2007 at 10:15 AM.
I brainstorm with my multiple personalities.