I haven't taken econ classes in a very long time.
The idea is premised on an economic equilibrium point between supply and demand. Increase the supply through lowering taxes or barriers to production, and the equilibrium point will be moved further out on the graph. The price will be lower so more people will be able to consume the item. More people will be working producing the item (full employment).
My Keynesian macro professor showed graphs on how the theory was bunk. Of course, I don't remember them. If anyone could provide supply and demand graphs on how the theory is valid, I would appreciate it.