Generally, any form of agreement between two players that harm others is against the rules. That includes checking a hand down. In fact, most casinos DO NOT allow game-play talk in a pot with more than 2 players to prevent some sort of communication between the two.
Imagine a situation where you have a 5h 6h, your opponent has Ah, Kh, flop is 4h, 8h, As. You are losing pretty badly cause you are basically a gutshot draw (4 outs). However, what if your friend has 4s 8s? Your friend is currently winning but the opponent still have two Ace, three Kings, seven hearts... That's 12 outs per street (almost a 50/50 winrate with both streets).
Ideally, in a competitive play, your opponent and friend should raise you out and isolate the AK vs two Pairs cause your draw is far too weak to call any real bet. However, in collision play, since you and your friend are friends... You might want to slow play and make sure you are both in the game. That way your friend wins 50% of the time and even if he doesn't, you might still win with a royal flush or straight. If you share profits, the two of you will net more gains with slowplaying.
It's EXACTLY this kind of collusion that concerns players so generally any kind of communication that could lead to collusion is disencouraged. Obviously, you checking blind to "indicate" you want to slow play is ok, but if you talk to your friend and signal what you might have is not.
The same exact concept applies here. Any sort of communication leading to a "work together" against another player is against the rules. However, most seasoned players are smart enough to recognize those kind of play and would do it naturally.