When white extends up, black used to respond at A, but…
White can cut at A to fight or simply play B to take the corner. Both variations are good for white.
So now people will exchange this hane first.
If white cuts, black can sacrifice the stones to gain thickness. This is good for black.
If white hanes back, black can just connect, and the white stone on the left side is getting alone.
If white pushes up like before, now the corner and the side become miai