Black Sheep Rengo 2003/Yose Tesuji

Normal endgame  

AvatarDJFlux: I am a weak player in all the stages of this beautiful game, and especially in the Yose, but one of the very few things I happen to know is that the exchange for in the diagram below, despite being Sente, loses points... ( is W174 in the BlackSheepRengo2003, is B175).
After that White can exchange for and Black gets 5 and half points of territory. I count half point at a because both have the same chance to play first at b.

Yose Tesuji  

The Yose Tesuji for White is in this diagram.

Black must answer at , then White plays ! is again forced to keep Black his two eyes, is another Tesuji.
Here White gets 2 and half points, the captured black and white stones cancelling each other. The half point has the same reason as above. Therefore this sequence would have brought White a whole three points!

RNGS: Why "must" Black play at 2? Wouldn't 2 at 4 work?

I apologise to the Yose theoreticians, but I don't know much about tally-oh, ' don't know much about hotness... I don't know much about coolness... (to be sung on a famous R'n'B song).

Dieter: Thanks a lot AvatarDJ ! I'll try to remember this from now on.

HolIgor: The difference is sente and gote. Therefore timing is important here. Since black cannot do much white can wait with this variation till 1.25 points is the largest move. Though, actually and are sente.