We hear a lot about the possibility of a 51% (more accurately, 50%+) attack on BTC, but I don't understand what the attack requires 51% of. Is it the miners, and if so, is it 51% of the total hash rate? How is that determined and propagated? Do I then, as a BTC holder but not miner, have no influence on this? Or is it somehow tied to the total number of BTC in circulation?