Why do some people throw their head back violently when throwing up?

I know this is a very specific and weird question but earlier I wanted a laugh and a stroll down memory lane so I watched some old prank and reaction videos, I encountered this one video where they pranked their mom into watching some 2 girls 1 cup (which was huge back then) and her dry heaving (I guess this is the word?) gave me a laugh but also disturbed me. who heaves like this? why? you're supposed to bend forwards but she does the exact opposite.