Nineteen point trick play
The extension at here is the famous nineteen point trick play (also known as the "eighteen and a half point trick play"). Typical continuations for this joseki would be at a or b: see 3-4 point low approach one-space low pincer press.
If White falls into this trick play, Black will give up nineteen points in the corner (hence the name "nineteen point trick play") but get a huge advantage anyway. Read on!
and are is taken in by the trick play. before is another way to lure White into it but her correct response is different.
The sequence up to results in a huge success for Black. White has 19 points in the corner, but Black has such a thick wall outside that its value is much bigger than White's corner. Besides, Black a is sente.
(KataGo thinks White loses 12 points in this sequence)
Instead of in the continuation diagram, White should play here. While Black still gets a squeeze, White can take sente here.
In this variation, White dodges with a hanging connection, Black forces before connecting and attacks the marked stone.
See also Ryan Li's video discussing it: https://youtu.be/7dXWFlQA2P4?t=207s