My very reliable sources tell me that Redbull have a special lining of ultra high density solid fuel coated inside the exhaust pipes. It it aslo rumoured that RedBull also have higher than normal levels of hydrogen carriers in their fuel.
Sebastian Vettels sets his engine to "G-9", this is a special engine map that allows semi-lean engine cycling and post inginition to create very high exhaust gas temperatures. The solid fuel coating on inner surface of the exhaust pipes activates and evaporates. The fuel reacts with the excess air in the exhaust gas stream. Though burning at a moderate rate, the explosion from the solid fuel air mixture is still expansive enough to increase the energy of the exhaust gasses three-fold.
The fuel is only enough to be used for a few minutes. But with low speed corners, where exhaust blowing is required most, only a small portion of the lap, a few minutes of off throttle is more than enough for Vettel to blast out of corners with a 40 point down-force advantage.
It is said that the number 1 RB-9 car has exhaust pipes that are 5 kilograms heavier than normal. That is how unbelievable dense this slow burning rocket fuel is! Some fuel and balast is sacrificed to make suit. And that is why we see that the RedBulls qualifying advantage is not the huge one second gaps we were using to seeing in 2012.