White played an interesting sequence on the left now. What could it've been? Reuven
The tesuji at a is (Well not right away). Because it almost makes miai of killing the two groups.
If Black protected the upper group with b, White would play c while laughing in an evil manner.
However if Black protects with d... Wb Be Wf Bg... Black needs to win the ko twice to live.
Because of this White started with b in the game - White is a 5d on wBaduk but it's nevertheless amazing for a game at such speed. Reuven