All you need to know about Carpenter's Square - 27
Can secure this corner?
appears to be important, but can only start a ko here.
has the option to fight an alternative ko. This way white takes the ko first which is a slight improvement.
Making / exchange first is the correct order. Then white can come back to the key point and kill the corner - black cannot play 'a'.
Conclusion: Diagram 27.4 is a simple and elegant solution.