Wait for the next disc, it'll show the first time Ed does it... It's more of an ability he came by. It seems to come from chance, luck, or phenominal talent right now.
As for the Preist, the philosophers stone acts as the circle. It is a replacement/tool that greatly increases his abilities. Later, they find out that these philosopher's stones can allow non-alchemists to use alchemy, as long as the person uses a certain tool.