"Belly attachment" leaves White no way to link up, reduces liberties and threatens to destroy the eye in the corner.
If White tries to link up with , she is caught in a connect-and-die.
If White tries the double purpose play , then the atari at prevents the first line connection in sente. If White continues at one of the a, then Black at one of the b keeps ahead in the semeai between the two two-stone groups.
either does not prevent the first line connection ( at one of the a), or does loose the semeai between the two two-stone groups ( at b).