Is breaking your neck an instant death?

No, but if you don't receive medical aid immediately, you could bleed out very quickly. It depends on if both arteries are cut, how deep, etc.

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Just wanted to add some really nasty facts.

During the French Revolution they chopped heads of the noble and wealthy.

The heads were shortly held up for a public display afterwards.

Some tried talking, blinking eyes, crying etc. for the following 30 seconds and up to 1 minute or so.

It is a nasty fast death, but not instant.

Instant death would include total halt in brainprocesses in a fraction of a second.