They do, though it's not cheating as such if you're not doing it knowingly. The more general problem is one of 'unauthorised information', and it's not entirely

They do, though it's not cheating as such if you're not doing it knowingly. The more general problem is one of 'unauthorised information', and it's not entirely avoidable. Sometimes a position requires an unexpected length of time to analyse, because of the cards you're holding. You're expressly allowed that, but the break in regular tempo leaks information (that you have a decision which is salient) to the other players. That's authorised for opponents to know, but not for partner. The rules provide various restrictions on the partner of someone passing UI to try to prevent advantage being taken, but it's not an easy problem to solve in general.