A endgame tesuji (Endgame Tesuji 1) used as an alternative to the second-line diagonal move.
is the hitting-under tesuji, often an improvement over the diagonal move of a.
If , Black draws back with . This will be two or more points better for Black, and assuming defends, is also sente, just like the diagonal move.
If White counters with the hane inside of , cuts, must give atari, then pushes in to what was White's territory--not a good trade for White. However, depending on the position could succeed, and reading it out is a prerequisite for playing this move.