4-4 point low approach low extension, 3-2 contact play on the slide

Not joseki  

is an example of a non-joseki continuation of this standard variation. This idea occurs also when is played at a. The joseki play is at b. Black's tenuki is possible, but the contact play is hardly ever justified

White's first option  

White's first good idea is to play here. Black will have to sacrifice the marked stone.

White's corner ponnuki is too good  

Generally it isn't good for Black to give up the corner this way. White lives quickly - the ponnuki gives good shape in sente, and White can now turn to the outside.

Special plan by Black  

For Black to play this way instead is just about imaginable in special circumstances. White presumably connects the ko immediately with , for a good local result. Black would feel that the marked stone was inefficient, normally. If Black urgently needed to defend, taking sente and then playing a might possible make sense.

White's second option  

In fact White has another way to play here, with and making strong shape on the top side. White has the choice of good direction here. That accounts for the rarity of the contact play in high-level games.

Charles Matthews