You will recall in deashi we teach to push uke on a backstep, which should either cause him to rock back and forth or take another full step back, either of which allows us to do deashi.
But what happens when you get the timing of the sweep wrong and uke's front leg is more weighted than you thought - when you hit deashi and his foot does not move?
In this case, you take a turning step (just like in walking kata) around uke's front foot to stand beside/behind him, then when he turns back to face you, kosotogari appears.
So, where is the kuzushi in kosotogari?
- Timing - after an attempted deashi when uke's front foot is weightbearing.
- Direction - there is really not a lot of push/pull in this, but tori turning the corner invites uke to turn the corner after.
- Unintended action - uke had intended to stand still and weather your deashi attack. Instead he was forced to turn the corner to keep tori from slipping behind him.
There is the off chance that instead of turning to face, uke may refuse to move, but that is just dumb because that is inviting some much more awful things, like sukuinage or taniotoshi or uranage.