A speed transmission apparatus containing a first rotary crankcase, a double planetary gearset, a first rotation transmission arranged at an intermediate output of the first gearset, a second rotation transmission arranged at an intermediate input of the second gearset, a first intermediate shaft arranged in one of the first and second transmissions, a first electric compression brake acting on the first intermediate shaft and capable of modifying a rotational speed thereof, a computer electrically connected to the first compression brake, and an electrical connection capable of being provided between the first compression brake and a second compression brake whereof the effect is to accelerate the second intermediate shaft by the braking effect exerted on the first intermediate shaft by the first compression brake.