Double hane example 1
If White resists by capturing a stone, Black captures the marked stones and kills the corner as well. As in the diagram above is such a powerful play, White has actually made a mistake before and should have extended to above himself.
If White defends at , (or a or b) connects and gains major influence towars centre and side. The idea was not to kill White in the first place. That's just a punishment if white tries to get too fancy. The idea of this maneuver is to seal white into the corner.
Of course, White can still play backtrack to here. Now the situation depends on the ladder at a. Black has alternatives at b (if there are friends down the side) and c (a loose ladder follow-up to expand the influence.