As we saw in the work about Wings, when we studied the air flux around an inverted wing, the force of negative lift created by the interaction of the wing with the air flux that crosses it, is due to the difference of pressures of the air fluxes that cross over and under the wing, provoked by the different speeds that those fluxes travel those surfaces, due to the asymmetry of the wing’s profile.

In the Formula 1 bodywork case, and imagining it as an enormous inverted wing, this differential of pressures is the result of the acceleration of the air flux that is channelled to pass under the vehicle, between the under floor surface and the track, and that flows faster than the air flux which passes over bodywork, which has a short distance to do.

As we can observe in McLaren MP4/20-Mercedes of 2005’ profile (drawings 1 and 2), probably the most effective of that year’s Formula 1 cars, the air flux, after crossing the front wing (m), will be guided, not only by the wing’s own drawing, with their flux deviators, placed in it’s inferior face, like also by the chassis and the flux deviators (j, l) and splitters (i) placed between it and the front wheels, to, for the side pods leading edge, have access to the step plan (f), and, under the chassis(k), to the reference plan leading edge(g).

The air flux will travel the whole space created between the under floor surface and the track surface, with these two levels, the reference plan and the step plan, separated by an unevenness of 5cm and of round boards.

This space is going to be narrowed light and progressively as the flux of air approaches the back axis, because of differences in the regulation in height of the front and rear suspensions, to reach the minimum height immediately before the beginning of the rear lateral diffusers (e).

This area, located about 40 cm ahead of the rear axis, acts as the throat of the Venturi tunnel, that we analyzed in the Wings article, and it is the place where the air flux reaches the biggest speed, and consequently, the place where the pressures are more negative, meaning that, the place where the largest load of negative lift is exercised on the bodywork.

After crossing this throat the air flux slows down, and the pressure increases, softly, in the passage by the rear diffusers, until joining, in the trailing edge of this enormous "wing", located, in theory, in the trailing edge of the inferior rear wing (c), to the air flux that passed over the bodywork.

The rear diffusers correspond to the divergent duct of the Venturi tunnel, and have as main function, allowing air flux, accelerated in it’s passage by the throat, retake, in a progressive way, the speed and  pressure levels of the free air flux that circulates around the vehicle.

Since the under floor surface is a flat one, and the distance to the track pavement is very small, with the added difficulty that the reference plan, with the "skid block"(h), must reach the level of the rear axle, the inclination that the technicians can use in the under floor is quite limited.

From here it resulted that the technicians needed to discover another additional way of accelerating the air flux at this level that could increase the aerodynamic load created in that area.

 It is known that the need sharpens the creativity, and like this the famous "step" was invented, in the beginning of the rear lateral diffuser.

When the surface that limits a certain air flux presents an angulations’ of 900 opened outside of the previous trajectory, the air flux is forced to do this straight angle, by suction, and, by doing it, suffers an immense acceleration.

This air flux behaviour has been used, since 4 decades ago, in the racing cars, through the use of the so called "Gurney flaps". These are small aerodynamic devices, in straight angle, that, when applied on the trailing edge of the wings, they provoke, due to the effect of additional acceleration of the air flux that passes under the wings, when being forced to do the straight angle of the "Gurney flap", a larger stability of this air flux, that stays adherent to the inferior surface of the wing for more time and in larger attack angles (angle created between the free air flow direction ahead the wing and the imaginary line that unites the leading edge to the trailing edge of the wing: the bigger it is, the bigger will be, at least theoretically, the aerodynamic load generated by the wing). With the "Gurney flaps" it is accomplished a simultaneous increase of the aerodynamic load generated and of the air flux stability that passes under the wing.

When applied on the rear limit of the step plan (drawing 5-b,e), as we see in the rear lateral extremities of the under floor surface, and in the rear limits of the splitters of McLaren MP4/20's lateral diffusers, these "Gurney flaps" will provoke an additional acceleration of the air flux in the passage under the rear extremity of the under floor surface and under the splitters of the lateral diffusers, increasing, consequently, the negative aerodynamic load in that area, in a similar way of what happens in a conventional wing.