What does the 9 do in Shithead?
Verdict: in the official rules, a 9 forces the next player to play 9 or lower. The 9 is not magic on the way in: it has to be played legally like any other card. But once it lands, the pile changes direction. Another 9 passes the demand along; a magic card dodges it. Plenty of tables play the 9 as a plain card, usually because their descent rule lives on the 7 instead.
Why we back the 9
Because it must be played legally, the 9's power arrives when the pile is already interesting. A 9 dropped on a tall pile with the next player sitting on a fistful of court cards is one of the great moments in Shithead. The 7 version hands the same power out earlier and cheaper.
Our ruling
The descent belongs to the 9. If your table runs it on the 7, or nowhere at all, that is a house rule.