How to modulate a marker with a square wave

[Jon1967us]

In the book Guide to Color TV Servicing, published in 1970, it states that one way to assist diagnosing poor video response is with a 15.75khz square wave, and that there are 2 ways of doing so. The first is to feed the signal into the video amp, after the detector diode, adjusting the amplitude and DC offset to match the video signal that would normally be there. The second method is to use a square wave to modulate a marker generator, which is fed into the antenna terminals. By analysis of the effect on the square wave form, one is supposed to determine if there is excessive or insufficient response of low/high video response. Im not understanding the second method here. How would it be modulated? By frequency or amplitude? How would this be helpful?