The trigger  

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!

The trick play sequence

Nineteen point trick play  

and are is taken in by the trick play. before is another way to lure White into it but her correct response is different.

Continuation  

is the real mistake in the whole sequence and should be played at . See below.

Conclusion - @  

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)

Descent  

Instead of in the continuation diagram, White should play here. While Black still gets a squeeze, White can take sente here.

Don't cover  

In the conclusion diagram covering at a is not the best option. Rather White should turn at and allow a capture two recapture one at a.

Dodging early  

White can dodge this line of play with this calm variation. KataGo advises an even stronger resistance: instead of , white can turn with at a followed by and a pincer with at b.

Dodging early (2)  

In this variation, White dodges with a hanging connection, Black forces before connecting and attacks the marked stone.