Ugly descent or ugly sagari is a tesuji used in some particular types of capturing races in the corner. The name "ugly sagari" was coined by HolIgor.
In capturing races like this, the ugly-looking descent like here might be the key to overcome problems with shortage of liberties. In this case, it is the only correct move.
puts up a stronger resistance, but the same clamp at still works. While is a tesuji for gaining liberties, White is still ahead by one move because the ugly descent prevents Black from making the approach move at a.
If plays on the 1-2 point instead, then can throw-in. White can only respond by creating a ko with .
This is an elaboration on why in the previous diagram has to start a ko. Because if captures the stone, then after the descent at , Black has the cut at which prevents White's approach at a.
There is also a belly attachment tesuji at , but it only results in a ko, unlike the unconditional capture achieved by the correct ugly descent move.