You fork a blockchain by linking new blocks on the end (with bitcoin.pdf forming an implicit start node they all kicked off from). Whichever of the forks persua

You fork a blockchain by linking new blocks on the end (with bitcoin.pdf forming an implicit start node they all kicked off from). Whichever of the forks persuades enough people to continue mining on it becomes the real blockchain and the people will slowly stop working on the other forks. The shorter blockchain will never find enough people to mine it when there's a larger more lucrative blockchain to mine.