The "mx" stands for motocross, which relates to me, and the 3/4 circle is where the play on words comes in.
Here is how:
A friend of mine once asked why the number on my dirt bike is "58" and not "270". So I ask, why would it be 270? He replies, because if you are facing North, and turn clockwise until you are facing West, you completed a 270 degree turn. This makes sense because my last name is West.
Now, you get the picture.