I demonstrate how to perform an interaction contrast analysis in SPSS. This example is based on a 2x2 between-subjects ANOVA context. The main advantage of an interaction contrast is that you can specify the nature of the interaction on the basis of theory, which allows you, some would argue, to conduct a one-tailed interaction analysis - something that is not possible with a regular ANOVA.
I describe the nature of interactions in an accessible way in my textbook (Chapter 9): www.how2statsbook. Additionally, you should check out this video: https://www.youtube.com/watch?v=VZeJg_ynfTc&feature=youtu.be