4-4 point low approach, low knight's move block
If is already in place, is an effective means of blocking off the corner, applying a bit more pressure on White while leaving a bit more aji in the corner than the alternatives of a and b.
Without in place, can be played to deprive White of a base on the top in the case where White's position there is cramped, but contrary to what beginners might believe is not effective at making territory. In fact, it's a bit of an overplay, leaving vulnerabilities at the 3-3 point, and the Cut across at b, especially if White gets a move in place later around c.
Although it may seem submissive for White to play at and cede the entire corner, the White group up to is very strong and is not vulnerable to attacks. White may have the possibility of building up a framework outside.
Usually Black will not ignore , because after and , White settles easily with Black's corner being weak. If at , then at a.
Continuing from the previous diagram, if connects, then White can play the double hane, and the tesuji at ensures White captures the two stones in sente. If is played at , then there is a serious aji at the cutting point at .
Alternatively, may just extend. In this case, White is treating the stone lightly, and may aim at a in the future. White also has the choice of blocking at b later to revert to the previous joseki. For Black, b is a very thick move, erasing much of the corner aji] and further depriving White of a base. Black might even move there immediately.
If Black does not respond to , is severe. gives Black a cramped corner, and coming out (such as at a) lets White slide into the corner. Therefore, in response to a move around has some urgency for Black.
(21 April 2005) Jared: I tried this in a recent game, but I think Black gets too much territory.
Alex: and are playable - at least, I see them in amateur dan games often enough, though I don't like them myself - but should extend further.
Harleqin: Recently I got a very good explanation on this from a strong player. White should just play at directly. Then he has many possible continuations, e.g. also on the spots of (continuation shown above) or in the diagram, of which the in the diagram is the worst. After protecting the corner with , Black can also omit in the diagram. From this it becomes clear that is aji keshi.
Alex: Yes, except that usually Black will answer by playing at . So if White wants this result, first is necessary. It depends on the situation.
Alex: I'm quite fond of the footsweep and use it a fair bit in my games. My opponents often choose to answer it by making a second approach at here. The - exchange was natural for me, but I then followed up with a the first time I encountered the double approach. Later, I did a search on GoBase and discovered that this pattern comes up once in a while in professional games, but the pro move is usually here. White can, of course, cut off , but then he ends up with two weak groups against one, so the fight usually favours Black. Personally, I'm almost always happy if I get to play this way.
Harleqin: This struck me as quite thin, so I did a GoBase search myself. As I suspected, is mainly played when Black is already strong on both sides, thus making miai of an almost devouring attack on either side. In other situations, Black will rather play 'a' or 'b'. In any case, a complicated fight will likely develop from this, and while Black is usually rather strong in the area if this situation develops, he also has to take care of some bad aji in this corner.
See also: BQM 146