Jumping out with the knight's move
This is from Iwamoto Kaoru-Kitani Minoru (B) 1953-10-15. The virtues of over Black at a, are that it is somewhat lighter (sacrificing the square-marked stone later might be acceptable), and also that it moves Black's group one line to the left, away from White's wall and closing down on White's stone on the side.
, rather than at b, perhaps requires explanation.
in answer to White's marked stone is always somewhat vulgar shape. Here Black has to give up two stones after . Next Black a, White b and Black has no attack.
This result gives White a potent ponnuki. Allowing White to play at can start a ko for high stakes. The stone then can't be considered well-placed.