Poker rules state that a hand is only required to be shown to determine a winner after all betting rounds have been completed...at the "showdown". If someone folds a hand prior to the showdown, they are not required to reveal their cards. Nor are they required by rule or by etiquette to tell what they had, if asked. Of course, it is never acceptable to show your cards while the hand is still being played out.
poker strategy varies from player to player as to the voluntary showing of cards, winning or losing. Some players will show to set up a later bluff, or to agitate their opponent into tilting. (A classic example of this was James Sousa at the 2004
wsop $5K NLHE Championship. Six straight all-in's with no callers, each time showing the table monster hole cards. Stole a huge pot from Thomas Keller when Keller folded to the seventh all-in. Sousa showed him rags.) Others staunchly refuse to ever reveal their cards unless someone pays to see them in a showdown. When playing online, count me among the "automuckers".
I've noticed lately on televised poker matches that in head to head play at the final table, one player will sometimes hesitate to turn his cards over at the end of the hand. If the other player shows first and wins, the loser will muck his cards. I haven't seen this ploy being objected to, but I was under the impression that both hands had to be shown at that point. Can anyone clarify this for me?