Amplitude modulation using a sinusoidal wave shifts the original spectrum in frequency by the sinusoids frequency. For instance, if there is a source with a square spectrum spanning -100Hz to 100Hz, and it is modulated by a 1000Hz sinusoid, it will have shifted the spectrum in both positive and negative frequency. Thew new spectrums will show two sources of energy from -1100Hz to -900Hz and 900Hz to 1100Hz.
To answer your question, we also should consider that their source signal is white-noise, and the carrier signal is a square wave rather than a sinusoid. A square wave, rather than having a single peak in the frequency domain (such as a sinusoid does), has multiple harmonics of descending strength. Modulating with a square wave will create multiple images of your source signal to appear, each centered around the square wave's harmonics and scaled by the half the strength of the harmonic.