This is a lesson I learned recently: I was watching one of the NAMT final games between Feng Yun and Jie Li, and Jie played a yose tesuji I was not familiar with. Later I found the same tesuji in a collection of pretty simple yose problems. Here it is: black to play in dia 1 below.
The instinctive move (for me at least) was to push on the second line, like black 1 in dia 2. White naturally blocks with 2.
After white 2 in dia 2 above, black unfortunately cannot play hane and connection on the first line in this case, see dia 3:
Oh, well, so dia 2 is the answer then - black keeps sente at least, he's happy. Unless he's behind by less than 4 points - in which case he had just played the losing move with in dia 2. That's right, black can reduce white by 4 more points by playing a smarter move. And that is... the lame looking kosumi in dia 4:
Indeed, black 1 (kosumi) in dia 4 above is the best move for black in this case. It is not easy to find if you don't have it already in your repertoire, because it is not an intuitive move: it looks like black is avoiding fight. But that is the reason it works in this case: there is no fight here, it's just about territory.
Normally white can play 2 in dia 5, but that doesn't help in this case, see for yourself:
Instead, white has to has to play all the way back to 2 in dia 6 if he wants to stop black. If you compare dia 6 with dia 2, you'll see why kosumi is 4 points better than pushing in this case.
The following two diagrams show why black cannot play ogeima here (which is a standard yose move)
No grand conclusion here - there is a limited number of standard yose tesuji, and this kosumi is one of them. We all need to learn it.