he can do that because that particular corsair is a good quality psu, able to deliver more than the rated 1200W.
Its probably close to the limit, but they could set the power targets appropriately to stop the cards hitting such high wattage.
No you are right, by any reasonable calculation of power usage this is running the power supply past its rated limits and asking for trouble if trying to run 4 x GTX 690 cards on a 1200W power supply.
If this is with a 130W CPU and a standard system of this size, I would allow for 1390W power draw.
Not wanting to run a supply past 80% of its rating, I would suggest a power supply (or supplies) with a +12V rating of at least 1740W or 145A.
As a single supply, that is only possible on 240V mains.
More common would be to use two supplies and power say two graphics cards off the second supply.
Thank you all for the answers.
So it's possible ( and safer ) to use two supplies ? Mr VincentP please can you link me to some threads so i can read the configuration of that setup ?