The July edition of RaceTech Magazine has an excellent, and timely article by Pat Symonds on the subject of exhaust-blown diffusers (EBDs).
Pat points out that, whilst the purpose of EBDs in 2010 was to increase the volumetric flow-rate of the diffuser, the primary purpose of 2011 EBDs, restricted by regulation to the outer extremities, is to create a curtain of air that inhibits the ingress of turbulence from the vicinity of the rear-wheels.
Symonds explains that the effectiveness of an air curtain is determined by its momentum flow-rate, which in the case of an exhaust flow jetting into the ambient flow of the car, is proportional to the exhaust velocity divided by the car velocity.
(Incidentally, in terms of naturally-occurring air curtains, a good example might be the Antarctic circumpolar vortex, which concentrates ozone-depleted air over the South polar region, and drives ocean currents which prevent warm water from mid-latitudes mixing with the cold polar waters).
On the subject of hot-blown diffusers, Pat argues that hot-blowing on the over-run is difficult to achieve by retarding the ignition spark, hence hot-blowing may actually be most effective when the temperature of the exhaust alone is used to ignite a lean air-fuel mixture.
Intriguingly, Red Bull remain the only team to blow their exhausts under the outer section of the diffuser. If one were to hazard a guess at why they do this, the explanation may revolve around the pair of counter-rotating vortices generated by the outboard edges of the diffuser. These are reputedly responsible for keeping the airflow attached to the suction surface of the diffuser under conditions at which the airflow would otherwise separate. By blowing beneath the foot plate of the diffuser, could Red Bull perhaps be using their exhaust flow to enhance this effect?