sake bottle shape, textbook example
here is a 'tempting the opponent to cut' play. is a standard shape. In relation to making a double peep next at b, it is correct. Also one sees this kind of diagonal in other cases, for example easy way out of a double kakari, when the immediate threat to cut doesn't work.
White shouldn't play at and cut off , because bad things will happen to her stone on the left side. So then , and is honte: good firm shape. Black will welcome this if he can attack on the upper side.
Traditionally go teachers have discouraged this sake bottle shape. It is less obvious that it works well, analysing it as functional shape, or in terms of attack and defence. It has been played by a couple of top players, notably Ishida Yoshio who is quoted in a Go World article as saying that it really doesn't lose anything.