This is one of the standard methods for investigating sound production in vertebrate animals. You can also hook up electrodes to the muscles surrounding the vocal folds to study how they are involved in modulating the sound.
The trachea functions as a resonant structure. By pulling on it, he is increasing the volume of that structure and lowering its tuning.

I'm actually taking a stage voice class now and interestingly, female voices are typically higher because the vocal folds in women are thinner and longer, while in men, they are shorter and thicker; hence the 'deeper' voice.