I played a charity tounery a couple of weeks ago. When it came time to SD, one guy had a habit of only showing one card at first and only showed his 2nd if he really needed to-- ie, board was xxJJx and he held a J, he'd flip over the one J of his hole card, then flip over the other if kickers came into play.

I figure it's fine within the rules if he other guy in SD just nods his head and mucks since he's conceding the pot. But is there an actual rule on this? Just curious since I don't recall seeing pros do this on TV.