So, pitching with the vocal is basically about them vibrating together at different speeds. So the vocal folds basically vibrate together to create sound. They can stretch and thin in order to be able to vibrate faster, and when they are vibrating faster, the sound is higher.
There are various different thicknesses that the vocal folds can vibrate at – sort of like – the same speed and that can contribute to the overall – kind of like – feeling of the sound as well.
But for simplistic terms: as we get higher the vocal folds get thinner and therefore they can vibrate together faster.
Something that I find usually helps my students when I’m explaining this is explaining that as we’re getting higher, because the vocals are getting thinner, you don’t need to push (like) air at them because they actually need (like) less pressure to keep moving.
So as you glide up, (does a pitch glide) don’t push air, almost pull back, and just allow the sound to move.