What are the steps of an enzyme cascade?

I have a midterm on Wednesday in my Perception course and our professor told us that there would for sure be a question about the enzyme cascade. He isn't the best lecturer and so his thoughts were quite scattered when he was describing it to us in terms of how an enzyme cascade can explain how the isomerization of just one visual pigment molecule can activate an entire rod receptor site. I guess I just need as much information as I can get so that I'll be able to say enough about it for the short answer portion of the exam. I probably won't understand most of what you're saying but I will memorize it anyhow.

I hope this makes sense. It barely makes sense to me (I'm terrible at biology).