Wedge - four-space extension

The four-space extension  

An extension as far as is interesting here, but provocative. Black may well invade.

Perfect shape for White  

Now White's stones are perfectly placed.

Black invades  

Therefore it is natural enough for Black to invade, and try to make the marked white stone into a target for attack. as shown is popular in pro games: a is also seen, while b was played in older games.