If you're doing partial suspension, you'll pass out if the rope presses on the carotids, which is why you need to know where they are so you can position it above them without compressing the windpipe.
If you get a sort of fade to black sensation, you've got the carotids.
If you feel your head swelling, you've hit the jugulars. You'll still die but it's a grim way to go. Stop and try again.
This is the most common reason why partial fails. The jugulars, disposing of deoxygenated blood from the brain, are easy to access. The carotids are hidden deep as they carry fresh oxygen *to* the brain. That's why they're so important in life as well as death.