A powertrain includes an engine with an engine output member and a transmission with two motor/generators and a transmission input member operatively connected to the engine output member. A control unit is configured to selectively cause a device to resist or prevent rotation of the input member in response to the existence of certain conditions, such as the engine not supplying torque to the transmission and the transmission being in reverse operating mode. Resisting or preventing rotation of the input member improves transmission performance when only the motor/generators are supplying torque to propel the vehicle. A corresponding method is also provided.