Backdoor
also called runner runner
A backdoor draw is one that needs both the turn and the river to complete, so on the flop it adds a little value to a hand but not enough to play for on its own.
The most common one is a backdoor flush draw: three cards of a suit after the flop, with two more still needed. With ten cards of that suit left among the 47 you have not seen, the chance both the turn and the river bring one is 10 in 47 times 9 in 46, which is 4.2%.
That 4.2% is small, but it is not nothing. It is often the difference between a hand you give up on the flop and a hand you keep for one more card, because it can turn into a real flush draw and gain nine outs.
an example
your cards
board
You have no pair and no flush draw yet, only three hearts in total. A heart on the turn gives you nine outs to the best flush on the river. Two hearts in a row completes it straight away, and that happens 4.2% of the time.
This example is made up. The real one is in your own hand history: find backdoor in your own hands and see how often it came up and what it cost you.
related terms
Outs
An out is a card still in the deck that would give you the best hand, and counting them is how you turn a draw into a nu ...
Equity
Your equity in a pot is the share of it you would win on average if the remaining cards were dealt out and nobody folded ...
Semi-bluff
A semi-bluff is a bet or raise with a hand that is probably not the best right now but can still improve, so it can win ...
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Knowing the word is not the same as seeing it.
Forty hands against a bot, no account, and your own hand history at the end.