How does xboct do this?

I saw that his Hero attacks the creeps automatically despite he telling him to go back.

you can see that at the 12th second.
http://www.youtube.com/watch?v=A1AdWqWZrts&feature=youtu.be

I think it goes like this, Xboct tells then hero to go back and the cancels the command by pressing 'S' (or the predefined hotkey) and then the Hero auto-attacks the creeps. But how does he give the command to auto-attack?

A attacks the nearest target.

I think he used the attack move command better know as " deny" command. If you click a space and not a unit your hero will attack all enemies near it than got to the space you clicked.

This was it.

1.Right click back.
2. You click 'a'
3. Left click on ground
[Ur hero attacks nearest living organism]
4. Three first steps happen in a very short time. Repeat.

Cool that's how you do it, but I am not sure what's the reason? Like other creeps don't hit you cuz u go back?

You get hit the same. However he was slowly moving towards the next camp so he would have to walk a slightly shorter distance to reach the next camp saving a few fractions of a second.

If you watch closely, its a green x to move, then a red x straight after. The red x is attack move. (A - left click ground)
Its a form of animation canceling