The athame is an incredible
tool for directing
power within the
circle. One of the reasons for it being
double-egded is because not only can the athame be used to send
energy out to create the circle, but it can also be used to
collect that same energy to
banish the circle.
Case in point:
In order to cast the circle, I would take my athame and walk around my altar 3 times in a clockwise direction, otherwise known as deosil. This movement sends energy out from one side of the blade.
In order to banish the circle, I would walk about my altar in the opposite direction, that is to say counter-clockwise or widdershins. This movement collects the energy that was used to create the circle with the opposite side of the blade.
The athame can also be used to cut a doorway in the circle in order to leave it for a short period of time.
The athame should not be used to do any actual cutting, however. Wiccans sometimes use another blade for any actual cutting within the circle, and it is usually a white handled knife called a bolline.