Wat houdt een 51 procent aanval precies in?

Hier wordt het mooi uitgelegd:
http://learncryptography.com/51-attack/ Het komt er dus op neer dat iemand die over 51% of meer van de totale hash snelheid van het bitcoin netwerk beschikt, het netwerk kan ontregelen. Ze hebben niet de totale controle over het netwerk maar kunnen wel bijvoorbeeld: "They could prevent transactions of their choosing from gaining any confirmations, thus making them invalid, potentially preventing people from sending Bitcoins between addresses. They could also reverse transactions they send during the time they are in control (allowing double spend transactions), and they could potentially prevent other miners from finding any blocks for a short period of time. That’s really about it – enough power to cause some serious mayhem (as that’s all stuff that isn’t supposed to be able to happen) but nothing that would seriously cripple the network – at least not immediately. They couldn’t reverse transactions from long ago, create new coins out of thin air (besides through regular mining), or steal coins from other people’s wallets."