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"Show one show all" means if you show one player your cards you have to show all players - the reason is that you must give the same amount of information to all players.
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Precisely. I used to get confused in the same way about SOSA -- but the "all" in question is all players, not all (or both) cards.
Unrelated to SOSA, some rooms have a rule that you must show both cards if you show one. This was discussed at ample length in the past seven days on this forum.
Robert's Rules:
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Show one, show all. Players are entitled to receive equal access to information about the contents of another player’s hand. After a deal, if cards are shown to another player, every player at the table has a right to see those cards. During a deal, cards that were shown to an active player who might have a further wagering decision on that betting round must immediately be shown to all the other players. If the player who saw the cards is not involved in the deal, or cannot use the information in wagering, the information should be withheld until the betting is over, so it does not affect the normal outcome of the deal. Cards shown to a person who has no more wagering decisions on that betting round, but might use the information on a later betting round, should be shown to the other players at the conclusion of that betting round.
If only a portion of the hand has been shown, there is no requirement to show any of the unseen cards. The shown cards are treated as given in the preceding part of this rule. (Emphasis in original.)
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