Is he firing off that right hind?

A good lesson this evening. I ride around thinking he could give me a little more from behind but never really do anything about it. My instructor told me tonight I needed to goose Blu with my right spur since he was blowing me off. So I did and he swished his tail, swapped his leads behind and swapped them again then I had a most terrific ride. My endurance was great and we worked it. Why is riding straight such a mystery? I think I need to change my focus a bit ... I need to start thinking more about what is happening behind me and stop worrying so much about what is happening up front.