Here Black has no choice about continuing to push, because White[1] at a is a very good bend. Black should try to get ahead (3 at a or b), as soon as it is tactically safe to do so.
This is a bad situation for Black, because White can make the decisions, for example by playing hane at instead of
, or with
at a.
The general phenomenon is called a pushing battle.
Pushing from behind is called kuruma no ato-oshi (pushing the cart from behind, 車の後押し) in Japanese.
[1] (Sebastian:) or
? Also, what does "has no choice about" mean here? No other choice for
?
Black can choose to jump with at
, or
at a, but that always costs the shape something in the way of potential cutting points.
White's bend
This is what is called a bend.
If White adds here, there may be no standard term in English for this sort of jumping down and away?.
This is not in general a recommended sort of suji; there are occasions where it's strong and can be called tesuji.
(Sebastian:) So do I understand correctly that this means: Black doesn't want to continue building this wall ad infinitum, so she has to end at some point. Since that point allows the bend, it still looks bad for Black. So do other alternatives, such as the jump down and away. Therefore, she should rather not push from behind at all.
Charles The main point is that pushing from behind is a position of disadvantage. The number of ways it can work out is quite large: my own researches haven't led me to any definite conclusion. (It was once suggested this part of the game could be simulated by a finite state machine!) But rather than trying to predict the future, which is sometimes fruitless in go, one can simply notice that White here is making all the running.