In a decentralized system, where no individual or group holds the majority of power, the only way to seize control is to gain a majority of like-minded individuals. This is the origin of the 51% attack – where the system needs a majority vote to reach consensus.
Although a 51% attack in a large distributed network, such as Bitcoin, Ethereum etc. is highly unlikely due to sheer size, smaller coins face a bigger risk simply because there are fewer participants, which makes it easier to manipulate.
This is why the website crypto51 was made: a collection of coins and the theoretical cost of a 51% attack on each network, it is not meant to encourage attacks, but to get people talking about the problem and potential solutions. As to how it works, the website explains: “Using the prices NiceHash [a Slovenian cryptocurrency cloud mining marketplace] lists for different algorithms we are able to calculate how much it would cost to rent enough hashing power to match the current network hashing power for an hour.”