The best Black can get is to connect back the two Black stones in sente, leaving the endgame benefit of capturing two White stones at a.
Note that in this diagram, both and are tesuji.
cannot Block, because can cut. After that, makes a snapback. Now White cannot live. ( at a, , Wb still lives, of course)
is dangerous. and eke out a ko, and the life and death of the entire White group now depends on this huge ko.