That's what it all comes down to doesn't it?
I'm inherently submissive; however, my wife is also sub. When we go out to play with others, we have a few friends who are quite wonderfully dominant with us both, but when we are alone, I play the dom (and here's the point), and I ENJOY it. Therefore, I am switch.
Switching is not inherent, like dominance or submission. Switching (and the act of being a switch), comes from the necessity or desire to fulfill a role outside your norm.
In direct response to delia: A normally submissive person who is directed to dom another sub is an alpha sub. A normally submissive person who volunteers to dom another sub is a switch.