Is anybody have a good explanation about that 51% attack?

By the time of the attack the WDC network had nearly 1 Gh/s of power on the lowest, this means that the attacker needs 1,* Gh/s of power and the network then raises up to 2 GH/s .

This is a lot of hashrate, i think, and a big manouver to only win 64.000 WDC...

Who is this guy? Having such power it a real threat to altcoins ....