[zenko]

I noticed that at least one very prominent pair involved in recent US National team trials plays Rusinow leads, but only from 4+ card suits. There are plenty of situations where defenders can deduct from opponents’ bidding AND his own hand was the lead in fact from 4+ card suit or not, but that information is not available to the declarer. I do not see any material difference between that and other types of encrypted carding, like for example signals that vary depending on holding even or odd number of cards in the suit bid by opponents, etc. That type of carding is specifically forbidden by ACBL, and US Trials were played under those rules too. So, what gives?

[ArtK78]

That is not encrypted carding. Encrypted carding implies that there is a code which can be deduced by the defense but not by the declarer (such as the number of cards held by each defender in a key suit, the length of which can be deduced by the defense based on the bidding but the declarer does not have the information needed to figure out the length of each defender's holding in the key suit). The code determines whether a particular defender is using upside down signals or regular signals. For example, suppose declarer's side used Stayman in the auction, and declarer, who winds up declaring 3NT, showed 4 hearts. As soon as the dummy appears, each defender knows how many hearts his partner has based on the assumption that declarer has 4 hearts. Hearts will be the code suit. Alternatively, suppose the defenders lead two rounds of diamonds against a 4♠ contract, and declarer ruffs the second round. Diamonds will be the code suit. The defenders have agreed to use upside down signals if they hold an even number of cards in the code suit, and regular signals if they hold an odd number of cards in the code suit. Once a defender has shown out of the code suit, the number of cards in each defender's hand is known to declarer, and the defenders then shift to their preferred method of carding.

In this case, at the time the opening lead is made, it may not be possible for leader's partner to deduce with certainty how many cards his partner has in the suit. Or, it may be that declarer has the information as well as leader's partner. There is no code which has to be deduced - only the length of opening leader's suit.

[cherdanno]

I assume you are not referring to Meckwell? My understanding is the play Rusinow from known length. I.e., when opening leader has shown length in the suit he leads during the bidding.

[awm]

There is some fuzziness about what encrypted carding actually means.

I think the main idea is that the "key" to the encryption is something not particularly relevant to the suit being played. So for example, if we agree to play either rusinow or standard honor leads based on our length in the suit we are leading this is not considered encrypted, because the "key" relates to the suit being played. If we agree to play either rusinow or standard honor leads based on length in some other suit (perhaps a suit where opener's exact length is known) then this would be considered encrypted leads.

Note that it is quite common to play rusinow leads but to lead high from a doubleton honor (so K from KQ but Q from KQxx). This is just moving the line from 2 card suit vs. 3+ to 2-3 card suit vs 4+.