This is from Absolute`s website:
"At the end of the last betting round, all players who are left in the hand have a showdown in which the player with the best hand is awarded the pot. It is the obligation of the player who made the last bet or raise (not call) to show her hand first in the showdown.
All cards in a player's hand must be shown face-up on the table to be awarded any part of the pot.
The other remaining players then show their hands in a clockwise fashion. However, the other players do not have to show their hands; if they know they are beaten, they can just concede the pot. Remember that not all hands will reach the showdown stage; very often, a player will win the pot because all other players have folded their cards.
In the interests of efficiency and speeding up the game, a player who is reasonably certain he has the winning hand should turn over his hand immediately, regardless of the order of showdown. If a player does so, then other players at the showdown who can beat that hand should also turn their hands over immediately.
If after final action a hand is shown to one player at the table, then any other player at the table may ask to see the hand, and it shall be shown. If during the course of play of the hand a player flashes his hand to another player before mucking it, any player may ask that the hand be shown to all at the table, but it shall not be shown until after the end of the hand. "
Which is pretty much exactly what Welly said.
The only situation not mentioned in this quote is what happens if every one checks on the final round. I feel certain that Welly is right about that also - showdown begins with the player in the small blind position.