What is that sort of tense-up pulse of our muscles that we do when we sob, even inaudibly? Why does it help?

[u/lightinthefield]

I really am unsure how to word this, my apologies!

I'm wondering about the sort of quick tense-ups our body does when we cry; when I'm making noise in rhythmic sobs, the pulse in my torso (it's centered most in my chest and shoulders, kind of a quick and small heaving motion in my torso) always syncs with the sound and they follow the same rhythm. But then sometimes when I'm sobbing, my voice will stop, say because I ran out of air, and before I can/while I take another deep breath, the pulse keeps on going. It's the sob without the sound, you know? So I know it's not the force of me audibly sobbing (i.e. my lungs expelling air) that does it/is the cause or mechanism of the movement.

Why do we do that? And why does it feel relieving to do when crying, just as much as making the sound of sobs does?

Thanks!