The diffuser is an area of bodywork at the rear of the car, although the term “Diffuser” is technically incorrect, it is the most popular term applied to this part of the car.
The aim of the diffuser is two fold, it works like a wing curving up and creating negative pressure under the rear of the car and hence downforce, the second action it fulfils and related to the first is that the low pressure it creates actively pulls air form under the car, this scavenging both produces a lower pressure area under the car and also acts to reduce the boundary layer, a boundary layer is the result of the air molecules next to cars body are actually stuck to the car, they travel at the same speed as the car and not the same high speed as the air and ground passing next to the car, this layer builds along the long flat or curved surfaces of the cars bodywork. The create friction and disrupt the airflow around the car, before the plank and stepped underbody were regulated, the cars were allowed to run as low as they could, this made the boundary layer under the a big issue, the boundary layer would thicken along the underbody and the static air would literally choke the underbody, effectively stopping the airflow under the car and reducing downforce. The diffuser was used to pull the air under car more effectively and reduce the build up of the boundary layer.
This effect is reduced with the higher underbody heights regulated since the 90’s. However the close proximity of the pkank to the ground the need to produce as much downforce as possibel for the least drsag makes the diffuer massively important in the cars aero set up.

Simple Diffuser Design
To understand how a F1 diffuser works it helps to simplify the design. The diffuser can be broken into three parts, the centre and two side channels. If the geometry of the design is broken into triangles then we see from the image six 3D triangles (wedges).

The inner and outer channels each take up two triangles, the rules state outer channels must end at the axle line which is at about half the total length of the diffuser. The centre channel must have a flat bottom from the start of the diffuser to the axle line, from there the channel can extend the full length.
The simplest diffuser would have similar shaped channels and no interaction between the inner and outer channels. This would produce a diffuser with very short length channels.

Diffuser development
Teams realised they can start the central channel at the start for the diffuser and provide a shadow plate to meet the flat bottom requirement. To feed the centre channel the outer channel is split into two (or three) the inner channel has no fence separating it from the centre channel. There is then a fence between the inner and outer side channels. To provide a higher energy flow into the start of the central channel the inner side channel is directed towards the centre of the car and no longer is longitudinal to the car, the outer side channel is also angled inwards to match the angle of the inner side channel. The result is the centre and inner side channels are effectively doubled in length. There is also a resulting triangular area of the floor either side of the outer side channel, this is usually left flat or some teams form it into a small ramp to increase downforce.

Stepped underbody
The second complication on the diffuser comes form the shape of the underbody of the car, the area under the car is regulated to form two flat bottoms. The stepped underbody was introduced to reduce the effectiveness of the diffuser and the plank was introduced to prevent the teams running the very low ground clearances to enhance the effect of the diffuser.

The lowest surface under the car is the reference plane this runs along the centre of the car and must meet a minimum width and length, the reference plane starts behind the front wheel sand extends to the rear axle line.
Running along the reference plane is the regulation plank, this simple sheet of plywood must meet minimum thickness. Length and width The larger higher area of floor is the step plane, this is raised 5cm from the reference plane, this extends from behind the front wheels to ahead of the rear wheels. The transistion between the step and reference planes must be a continuous vertical surface.

The teams converge the sides of the step towards the rear of the car into a point. The edges of the step join into a Vee just before the rear axle line. This converging speeds up the air flow along the vertical edge of the step. The flow along the vertical edge of the step is very poor, the teams use the shape of the end of the step plane to improve the flow and pull through the diffuser. The resulting Vee in the start of the centre channel is useful to accommodate the end of the gearbox.

Another development some of the teams use is another ramp low down in the centre channel, this additional ramp starts the axle line and as low down as possible, the aim of this is to pull air from the space between the track and the plank.

Teams use the exhaust pipes exiting into the diffuser to “energise” the flow, there speed of the airflow through the diffuser can be sped up by the high velocity gasses exiting the exhaust pipes when the engine is revving. This apparently free increase in diffuser performance was very popular in the 90s and was encouraged by bodywork restrictions preventing the exhaust pipes routing over the gearbox. As the performance of the diffuser grew and it became a larger component of the cars rear downforce, it became a problem that the effect of the exhausts blowing was reduced when the driver lifted off the throttle and the speed of the flow reduced, teams found this made the cars handling very sensitive to engine speed and slowly the practice was dropped first by routing the exhausts into less effectual positions in the diffuser and then later through the top of the sidepods using the Ferrari inspired “periscope” exhausts.
Three teams still blow the exhausts into the diffuser, Williams and Minardi blow the exhausts over the inner side channel, in fact Minardi have shaped the ends of the exhaust pipes to form the top of the diffuser channel, Williams flattened the bottom of the pipes to merge the flow of the exhaust and out the top of the diffuser channel. McLaren blow their exhausts above the shadow plate in the centre of the diffuser, there may be a few reasons for this practice, one of the main reasons is to energise the flow coming from the transition area of step plane, McLaren use the exhausts blowing against a knife edge formed between the pipe and the vertical step, this speeds up the flow, improving the scavenging effect ton this area, it also adds t the downforce generated by the central diffuser tunnel.

Rear impact structure
Recent years the FIA have introduced a structure that meets minimum cross sectional area, to form a deformable structure to cushion rear impacts. The tests for this structure have steadily increased and for the 2001 season a lot of teams had designed structures that needed to have a straighter path to the rear of the gearbox in order to survive the impact test. These structures encroached into the high centre diffuser channel reducing its effectiveness. Some teams flipped up the rear of the structure over the rear wing mount, leaving the diffuser channel free of obstruction, Jaguar in particular started the season with an obstructed channel and developed a new diffuser and rear impact structure to improve the aerodynamics.

Other design considerations
The teams need to leave a hole to insert the outboard starter mechanism, this usually results in small cut outs in the lower ramp of the central diffuser and in the Vee formed above the shadow plate. The effect of this is minimal.