SeeThruHead

so from what i can tell cleetus uses a hydraulic handbrake on the rear. his two step is automatically enabled when the ecu detects no wheels speed.

to bump in he's using pressure on the hydro brake and letting the clutch out. he may or may not have a linelock on the front, i kind of doubt it but could be wrong.
you could create a bump button the same way that transbrake cars have if you had the time and inclination.

basically the button would disengage linelock/rear brake for a very short time. you'd have to be slipping the clutch while you press it.

EDIT: I dont actually understand the physics of why you'd want a linelock except to heat up your tires (or for bumping in for staging like cleetus does)
for brakeboosting it makes sense because revving the engine with the clutch in will reduce your roll speed.
for starting from a dig. if you engage linelock you'd have to slip your clutch to "load" the rear wheels while still having enough engine speed to build boost. also i remember from high school physics class that once something is slipping it's coefficient of friction is different. static vs kinetic coefficient. that might also apply to slipping the clutch not sure sure.
in an ideal world i think you'd want to engage the clutch instantly at the perfect rpm for the surface you're on. and never slip the clutch. But I have no practical experience with this stuff just keyboard jockying so could be completely off base

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