Flash:
A universal motor has the field electromagnet coil and the armature electromagnet coil in series.
The E unit, when cycled, takes one or the other of those coils (the field or the armature, depending on the type of motor) and reverses it with respect to the other coil. That's what makes the motor run in reverse.
If you reverse the input DC to a stock motor, (assuming that you don't want to use the E unit) both the field and the armature magnets get reversed, but they stay the same polarity with respect to each other. So, the motor will still run in the original direction.
By putting a full-wave bridge rectifier between the supply and the field, when you reverse the track polarity, the armature polarity reverses, but the field polarity stays the same. So, the motor will reverse.
