Black 12 is tesuji. White 19 starts a ko. If Black a, White b. This is the solution.
This variation is better for White, because there is no ko.
Timing is important. If White tries 5 to make alleged miai of the left and the right, Black sacrifices and squeezes ...
... leaving one eye for White.
Q: Why not start with the key point White 1 here?
A: Because Black plays 2.