Running form question about arms?

I met with a running coach yesterday who pointed out that I cross my arms (diagonally) when running and suggested I would save a lot more energy if I ran with arms moving straight like a pendulum. I ran 5.5 miles this morning (including 1 mile warm up). Initially it was weird to get used to the new position for my arms but then it became more natural. I felt that I was running much more fluent. Has anyone ever made this change before and also how does it work exactly that the running becomes easier when the arms are moved in a straight way rather than crossing?