It was a new variable added in v1.250
"Drafting can now affect aerodynamic balance. The new variable is [BODYAERO]DraftBalanceMult=1.0. Note that this balance variable was functionally equivalent to 0.0 in previous builds, meaning that the draft was calculated once for each vehicle and affected all aerodynamic components equally. Now the draft is calculated at the front and rear of the vehicle, and components are affected based on their location in the draft. This multiplier allows you to exaggerate or remove the effect."