This looping input is for a reference signal directly out of the master sync generator. Don't take me too literal on this last statement. Using a DA, or possibly a second system sync generator will most likely work. Whenever I have had system problems and the Waveform display didn't seem right, I connected to the master generator directly. The signal connected to the reference input will depend on the particular device you have.
Most waveform monitors require composite sync as the reference input.
Our combination waveform monitor and vectorscope requires Black Burst, because the vector display needs burst. I have cheated, in a pinch, and used other signals to reference these units, but I don't recommend trying it on your own. This is used in conjunction with full scale studio television cameras. Most video cameras in common use today do not use this feature. The last connection on the rear is labeled Video Out.