forces to save the corner (White can get a ko to kill it). Then White plays tenuki here.
The point being that and are sente plays now, but don't break through, an example of dropping back. Nothing difficult, with in place; but you have to see it in a real game to feel the force of it.
Assuming this came about as a result of the play here, is required. After , can White take still take sente?
This is much more profitable for Black (next Black a and White will have to take the stones); so that in practice White may not be able to take, should play reverse sente.
In the real game the play was a bit more like an endgame tesuji.
The way the problem was set makes this / exchange first look better.