What fundamental physical concept does 'Kamm's Circle of Friction' (Kammscher Kreis) explain in driving dynamics and driver instruction?

1. The engine torque curve across different gear ratios
2. The total available friction force between the tire and the road surface, and how longitudinal forces (braking/acceleration) and lateral forces (cornering) compete for this finite grip limit
3. The electrical charging cycle of hybrid vehicle batteries
4. The aerodynamic downforce generated by a rear spoiler