When using a helper while backing, what should the driver and helper agree on before backing begins?

49 CFR 383.111(a)(6); AAMVA CDL Manual Section 2.2.4, p.2-8

A helper is only useful if the driver and helper have already agreed on what each signal means; improvised signals in the moment can be misread with the vehicle already moving. A clear, pre-agreed stop signal is the single most important one, since it's the one that prevents a collision outright.