Tewari Example 6

Hikosaka Naoto (W) - Mimura Tomoyasu, 2007  

From this Asahi newspaper page.

If White plays at a, Black can play at b. After long thought Hikosaka played to prevent Bb with sente.

Tewari  

However, if the play had gone - , the exchange, - would be bad. It strengthens Black and makes look good.