Its a unlocked feature once you have enough points

Narishma a day ago | parent | next [-]

How many points?

Here's a decent write up

	▲	minimaxir a day ago \| parent \| next [-]
		For posterity, you can only downvote comments, you can't downvote posts.
	▲	Narishma 19 hours ago \| parent \| prev \| next [-]
		It doesn't say anywhere in that document that you can downvote posts.
	▲	a day ago \| parent \| prev [-]
		[deleted]

iLoveOncall a day ago | parent | prev [-]

For comments only, not posts. Downvoting comments will achieve nothing when it comes to reducing the prevalence of AI spam posts.