The reason why this works is due to high nitrate content of spinach. Green vegetables in general are high in nitrate which is a key component for the body to create Nitric Oxide which is critical to getting and maintaining an erection (see my article under "Editor's choice" for a more in depth analysis).
Spinach and Tomato
Eat spinach with some tomato on an empty stomach (2-4 hours) after supper later at night around 11 pm-2 am.
I was hungry one night, and had nothing else to snack on. It caused an erection and horniness about 2 hours later. Repeated and had same reaction another night as an experiment.
I think empty stomach and later at night is key.