Missing parameter in "selectImage"?

In the code for Chapter 18 Intermediate 3D Animations the function selectImage is defined with 1 parameter as shown:
“func selectImage(selectedImage: ImageViewCard)” however when then this fun ion is called in “viewDidAppear” it is done without the parameter: “… image.didSelect = selectImage”

I know this compiles and runs, but I have not been able to find any documentation supporting this syntax. Can you clarify?