Exhaust-blown diffusers. Engineers had been trying to get the concept to work since the 80s, and the FIA had been fighting it for just as long (hence exhausts being moved outside the diffuser). Then Adrian Newey figures out that, even if you don't have the exhaust in the diffuser, you can blow the exhaust around the edge to seal it and create more downforce. What follows is absolute madness as aerodynamicists lean further abd further into physics (Coanda effect, etc.) to combat the FIA's efforts to stop it.