Pushing battles in joseki 4
White 5 is quite natural, but it also is slightly thin shape. Black would like to fight a pushing battle here to take advantage. Clearly Black at a is good shape, but the question is how to use tactics against White's weaknesses.
In recent pro games, Black plays out 1 to 6 before hane at 7. Playing at 7 immediately has been tried and seemingly rejected.
Black can try other things besides 2 here; but it isn't clear from past games that Black at a leads to advantage. White 3 is natural enough to cover the cut.