http://www.racecar-engineering.com/wp-c ... mercc5.jpg
Probably just some form leaf spring that deforms backwards as the splitter changes it's angle upwards which will then allow for much more flex so they can run the car lower without destroying the plank. It's setup that way so it can pass the specified FIA deflection mm/kg test and then after that it will yield.
Excuse the crude online paint drawing done using a trackpad, but I think you get the idea
Standard
Fia test, lots of force, small deflection
Exceeding that force by a small amount but lots of deflection as the material snaps backwards.
Easily tunable by changing the length, angles, and thickness.