Red Bull first tried a diffuser with shark teeth serrations along a gurney flap during Monaco practice. In that version, the serrations were at the outboard ends of the top of the diffuser only. This was raced in Baku and France. A more fully developed version with the teeth along the full width of the top of the diffuser appeared on Verstappen’s car from the Styrian Grand Prix and on Pérez’s from the Austrian GP a week later.
The whole aero philosophy of the Red Bull is that of maximising the static rake angle of the car (i.e. tail up/nose down). With a greater angle of attack, the underfloor generates more negative pressure, sucking the car harder into the track. At higher speeds (the downforce squares with speed) the tail of the car is forced down and along the straights it can thereby shed much of the extra drag associated with extra downforce. But there are practical limits to how far you can go with rake angle, one of which is mechanical (the angle of the driveshafts), the other aerodynamic (the tendency for the airflow through the diffuser to stall at low speeds/big ride heights). As the car slows the downforce bleeds off, the ride height increases and the airflow is at its slowest. So keeping that flow from just dissipating out the big gap beneath the diffuser becomes ever more difficult, and at some point it stalls and there will be a sudden reduction in rear grip.
Red Bull has managed to design its driveshafts to run at the extreme angles needed for a rear ride height of up to 150mm. But the aerodynamic problem was still imposing the rake limit before the mechanical one. What these diffuser serrations do – in combination with the gurney flaps which lift the outer airflow up, moving the point at which it merges with the airflow through the diffuser further back – is create multiple vortices of spinning air around the inner roof of the diffuser. This keeps the airflow coming through there energised at lower car speeds. In so doing it makes it feasible to run a higher rake angle. This increases the downforce created by the underbody, especially at low speeds.