The number at the table changes the game more than anything else does — it is what decides how good a hand has to be.
Also called Whiskey Poker, and in the card books Commerce. Three sentences of rules, and then the two ideas the whole game is made of.
Everybody gets five cards, and one spare hand — la viuda, the widow — is dealt face down in the middle. On your turn you do exactly one of three things: take the widow (your hand goes into the middle in its place, face up), trade one card with the widow once it is face up, or knock. Once somebody knocks, everybody else gets one more turn and then the hands come down. Best poker hand is irrelevant — the worst hand loses a life, and the last player still holding one wins.
Nothing is paid to the best hand. Only the worst hand pays, so the question is never "is my hand good" — it is "is my hand likely to be the worst one at this table", and those come apart constantly. A pair of sevens is a disaster heads-up and perfectly comfortable with six of you, because with six of you somebody is holding king-high and it isn't going to be you. Every extra player at the table lowers the bar. The strongest opponent here plays exactly that number and nothing else.
It is not a free draw. The five cards you throw away land face up in front of the next player, who may take the lot — and everybody after that gets to fish through them one card at a time. So the question is not "is the widow better than my hand". It is "is the widow better than my hand by more than my hand is worth to the people behind me". Somebody who has not noticed will hand out a pair of kings all evening.
A knock is not a pass — it starts the ending. Everybody else gets exactly one more turn and then the cards come down, so knocking early with a middling hand is a real weapon against a table that is all still fishing. It is also what stops a deal going round for ever.
Tap one of your cards and then one of the widow's, and they trade — that is all a trade is, so there is no button for it. Take the widow and Knock are in the tray, because those are the two things that are not trades. Until somebody takes the widow it stays face down, so the only way to see it is to be the one who takes it.