The definition is simple. Someone who is described as 'The Fence' is either so afraid to voice their opinion as to not hurt someone's feelings OR they simply don't have opinions and therefore agree with both sides. They are not 'sitting on the fence' as that implies they are still considering both sides of the argument. People who are 'The Fence' are so non-committal that they straddle both sides with such parity that might as well be hammered into the ground with some panels.
these are a very specialised group of people who are absent of any ability to make a choice in any walk of life.
they very often avoid making decisions and would much rather sit on ones fence, there it is said to be safe and free from confrontation.
if approached by large groups asking lots of questions at a fast tempo the fence sitters are often known to curl up into a ball or play dead.
chris: "hey john did u see that smogon stood over there on its own, the fuckin freak?"
john: "im saying nothing mate, i dont wanna get involved"
chris: "why have you curled up on the floor in a ball? oh yeah! your one of the fence sitters"
john: "stop asking me so many questions, i might accidently answer one"
The slogan of anti-centrism a comedic spinoff of extreme political ideologies. The slogan is calling for everyone considered an extremists to get rid of the metaphorical fence a centrist is on and cause them to pick a side.