Different perspective here. I don't work in the casino industry and never have. But, if I were tournament director here, I'd make the offending party sit out a round and be blinded off during that round. Not such an offense that I kick them out, but a penalty for collusion is in order.

Also, to Goodshoe's point, I actually believe that often times it is in NOT in your interest to check it down. As a random example, imagine you hit top pair as stated. What if you collusionary buddy is all in and hits his or her overboard to beat you? Do you really want an opponent to gain chips that should have been yours, had you but bet the flop? Does that help you get to first place?

Checking probably makes sense in certain situations. If you are in a free roll and are right at the money bubble - with everyone who makes the final table getting paid equally. Well, here I'd say a silent checking down makes the most sense. Here you are only trying to survive. But if you are playing a more normal tournament, where each spot is worth more money than the one before, you both need to knock players out AND accumulate chips. Giving free outs does not help accumulate chips. If you want to win it all, knocking out one more player is less important that building the biggest stack. The aggressive, go-for-it-all play is to bet top pair on the flop here.

Thanks for the comments. To clarify: I agree that checking it down may or may not be optimal. My objection is to explicit verbal agreements. One example: years ago, I was all in with three left in a SNG at the Plaza. Two players called, and player 1 said to player 2 (obviously loud enough for both me and the dealer to hear), "Just check it down". Obviously, I'd rather face one opponent than two. So my question really is: if I objected, would/could the dealer do anything to help me? If the dealer just penalized player 1 after the hand is completed, there is no point in me raising the issue. But if the dealer would go so far as to declare player 1's hand dead, it may be worth it to me to complain. I have no interest in being the table etiquette cop; but if calling attention to a rules violation can help me, I'll do it.

So different opinions I read here, but how do you think after all, is it correct to say about this situation to dealer and what effects to expect in response to your complaint? Can he stop the game and start it once again from the beginning?

I've been playing poker tournaments in casino's for over 10 years including the WSOP main event. In a tournament you're playing against everyone else in the tournament and not just the players at your table. There are scenarios where a non-participant in the hand is greatly affected by such an "agreement". For example, as you approach the money bubble, the short stacks (wo may be at other tables) are trying to hang on. If the other live player in the all-in is a short stack and could be at risk, the automatic check down is an advantage to this player as they are avoiding being felted.

If the players are overt in communicating the check down, you are within your rights to point this out to the dealer. If the dealer refuses to act, you can call the floor. However, a subtle communication (there are many) is "acceptable" and you'd likely not get any support from the dealer or the floor.

@amtorti Thanks for the comments. But I'd still like for somebody to tell me: what specifically would/should the dealer do if I raise the issue? Would he or she declare the hand dead for the initial colluder (the guy who said "check it down")?

@Goodshoe and @MattBob77 - According to the tournament director's association (TDA) rules:

64: Ethical Play - Poker is an individual game. Soft play will result in penalties, which may include chip forfeiture and/or disqualification.

I believe that most floors would allow the hand to play out -- after all, both the guy who said "check it down" and the person who agreed did call the all in before the collusion occurred. So, that agreement to soft play did not effect the action in the hand. However, both should receive at least a warning, if not a penalty, which could be sitting out a few hands, or an orbit, or even longer.

Dave

P.S. If you are interested, here is a link I I found to the TDA rules:
http://www.pokertda.com/wp-content/uploads/2011/01/Poker-TDA-Rules-2015-Version-1.0-full-longform-PDF-1.pdf

@Dap Poker Thanks!