This is about testing and debugging Video/Screen synchronization.
The problem is simple and common: You play a video file at a given framerate (eg 24 fps, 25fps or 29.97fps) while your Screen is refreshed at a different rate (eg 60Hz, or 75 Hz or 85Hz).
For the black video frames the signal will be zero during the whole cycle. For white frames it should be about 1V, with the start and end of each frame marked by a short (~1ms) zero.
This may give you an idea: smoothly playing a 25fps file of alternating black and white frames on a 75Hz screen: 3times black ; 3 times white frame with vertical sync.
The setup was done on one machine with two screens (one for monitoring, and the 2nd screen used as probe displaying the video). Using a single machine allows for software to measure the delay between a requested screen update and resulting signal (after compensating the audio-interface's latency). (Note that not all grafic boards allow DRI sync if two screens are in use)