The placement at is the tesuji; offers the strongest resistence. However, and are clever plays, because cannot connect at because of a shortage of liberties. takes away the second eye.
Black tries to expand the eye space, but the hane at destroys the possible eye at the corner. After , black dies from a straight three.
The simplest way of making ko is to hane at .
In the main line, if extends into the corner, then to forces a seki.