That is, if the decoder has never been addressed, it continues doing nothing. But if the loco is running, or has a light or other function turned on, it continues passing power to the appropriate wires. It continues through the device being powered, back into to DC - through diode B and out the AC2 connection. This explains how AC is converted to DC, but doesn t explain how it can operate a motor in both directions. The motor is driven by power transistors.
Transistors also allow power to go only in one direction.
T1 allows positive current to enter the positive terminal of the motor, while T4 connects the negative terminal of the motor to the negative terminal of the bridge rectifier. To make the motor go in reverse, transistors T2 and T3 are activated. Now we get to controlling the speed. Since there is full power on the rails at all times, there is also full power available at the power transistors at all times. If the transistors are turned on to pass available power, the loco would take off at full speed.