Not darkside. Its almost as lame a name as excalibur.
I think backside is better, it contains atleast some information. Situation, where your diabolo is under the string. Darkside is just silly.
"The dark side of the moon" makes a lot more sense than the backside of the moon. It provides more information. The dark side of the string is the one you cant see = under the string. Excalibur on the other hand I still never understood... why did we ever choose that word... who invented that?