A flux override module.
A flux override module was one of Emmett Brown's inventions. He used them mainly for diagnostic purposes when testing the DeLorean time machine, but when placed over the flux emitters that his alternate self had installed on the car, they allowed him to control the time circuits of another DeLorean remotely.
Each module included a small receiver dish, which could be aimed at a transmitter on the master vehicle to allow the time circuits to sync.