sprint

Basically, the shape is like an aircraft wing. The roofline with the sloping rear accelerates air over the top which reduces pressure and creates lift. At high speeds this will reduce stability... An active aero extension will redirect air upwards at the rear and disrupt the flow so it is slowed (increasing pressure above the car), at the expense of some drag – but preventing/reducing lift. The Jaguar F-Type has the same issue and this is the solution it uses.

Early mk1 Audi TTs had the same problem and they had to be recalled and fitted with a fixed duck-tail spoiler due to the car becoming highly unstable over 90mph – which was very relevant in Germany as their autobahns have no speed limits. In the next generation they brought in an active aero element on the tail of the trunk/boot.

The alternative approach for the Z Proto would either to ruin the shape with a fixed duck-tail or heavily manage the airflow under the car to accelerate the air under the car to overcome the lift (with downforce) or at least equalise it... But ground-effect aero is quite expensive to develop and manufacture... Hence why it's usually found on supercars.

Below supercars, most road cars generate some lift. The 2015+ 370Z claims to have zero lift (which is actually quite an achievement on a sports car in this sector). It solved it with a number of attributes, but significantly it had the fixed duck-tail spoiler. Hopefully, the new Z will have something similar to the Jaguar F-type instead.

I hope that helps.