Perhaps a stupid question but: How do combos work?

Ive played most of the game up to this point without out and out fighting but have occasionally bought a combo move.

Ive messed around with the settings and have arranged an air throw (rain? forget the name) into my combo.

On the rare occasions that ive fought it has happened a few times but I cant seem to consistently do the air throw over just the standard tripple slash combo.

What is it exactly that need to happen for the 'special' configured combo to go off? Is it a matter of timing? Placement? I dont get it.

Since no one what's to answer i'll answer it for you.
It depends on what you put it under. I have mine on a 4 hit combo. X,X,X,X,UP,4 I call it the Hayabusa drop.The combos are mostly XXX, Or X,X,UP. Sometimes X,X,Down for a trip. So..just try it.