During 2011 the teams were using aggressive off throttle maps in order to get a continued exhaust gas flow whilst the driver was off throttle using 'Hot Blowing'. Put simply when the driver lifted off the accelerator pedal the ECU continued to supply a small injection of fuel but cut the ignition, the net result is that the fuel still combusts due to residual heat but doesn't provide the same force it would if ignited. This results in an increased exhaust gas speed, effectively allowing a transition period whereby the driver isn't robbed of the rear downforce that the exhaust gases generate when the throttle is applied.
It is also possible to 'Cold Blow' which is essentially a leaning out process wherby you introduce less fuel to air ratio up until the point whereby you're 'Off Throttle' where the exhaust valves are still open, pumping nearly all of the air from the cylinders.
For 2012 the FIA altered the rules to try and restrict the teams using off throttle blowing to aerodynamically influence their cars. Firstly by restricting the exhaust outlets to vent to atmosphere atop of the bodywork rather than placed inside the bodywork above the floor. Secondly they amended the regulations in regard to engine mapping, torque deliverance, clutch position/usage etc. Although the FIA would love nothing more than to have Off Throttle mapping totally outlawed the engine manufacturers requested that a certain element remained in order to safeguard the engines. This obviously leaves room for the teams to pick holes in the regulations in order to further extract aerodynamic advantage from off throttle blowing, be it Hot or Cold Blowing to create downforce.
It is clear that the effect has been dramatically reduced from 2011 where the cars sounded distinctly different to how they do now under deceleration / cornering. (See the video below)
Melbourne 2012 and the teams were using Off Throttle Blowing but the sound is much more attenuated meaning the effect has been quelled. However some teams definitely still have a more audible off throttle blowing sound. In the video below (Thanks MrJokerman001) we can hear many occasions where both Red Bull and Sauber have a more distinct over run note (Start with Red Bull at around 0.11 and compare the deceleration to that of the Ferrari following. At 1.06 the person shooting moves to another section of the track and you can hear the difference between the Red Bull, Sauber and Williams over the next few minutes)
At Barcelona this time we can hear below that McLaren are utilising off throttle at around 0.22 in comparison to the cars that proceeded it.
Valencia: A video I have posted in the past showing the Red Bull use of off throttle in low speed corners
As the ability to control off throttle usage filters down through the teams we will see all of the teams pushing the limits imposed upon them to gain an aerodynamic advantage. As we can see in the video below from Silverstone (Thanks fraserjhamilton) Caterham were using off throttle blowing to assist.
Red Bull are now also using a Resonator / Expansion chamber in their exhaust which will not only add the benefits mentioned in my previous blog post: http://somersf1.blogspot.co.uk/2012/07/red-bull-exhaust-resonator-expansion.html it will attenuate the off throttle sound produced.
I have also previously talked about my theory of using Cylinder Deactivation in order to produce a more successful off throttle application, perhaps my intent behind talking about that subject wasn't clear enough but I'll try to briefly touch on this subject again now. (Having previously talked about cold blowing earlier in the post)
The advantage of Cold Blowing, Cylinder Deactivation & the Exhaust Resonator / Expansion Chamber would be that you would have a more tractable car. Lets look at the engine cycle to further explain my thoughts but we'll do it from the premise of a 4 cylinder engine to allow for an easier explanation.
Lets imagine that we are deactivating cylinder 1 & 3 that means you would normally have the inlet and exhaust valves closed and no ignition fuel sent to those cylinders during the piston stroke. However in order to cold blow lets imagine we continue to open the exhaust valve after the normal power stroke, the gas usually stored for the next cycle will be dispensed and cold blow. If this effect could be switched between cylinder banks after each cycle you would have cylinders 1&3 producing cold blow whilst 2&4 would produce the regular power cycle after this cycle is complete switch to 1&3 in the power cycle and 2&4 cold blowing. The largest advantage Red Bull would currently have here is the stored gas in the resonator / expansion chamber helps create a pressure drop in the cylinder sufficient enough to draw more air into the cylinder than would normally be the case when the inlet valve opens.
The teams are always intent on taking advantage of loopholes within the rules but in their quest for more off throttle effect perhaps we may see more cases of influence from other sectors to amplify it's effect.
(I have used amateur footage during this post as the footage available via the Broadcasters is always covered by commentary and onboard footage doesn't allow the sound to come across as well)