A probe bet is a turn bet from the out-of-position caller after the preflop raiser checked back the flop. The raiser’s check capped his range, so the caller seizes the betting lead with value hands and with bluffs on scary cards.
When the preflop raiser checks back the flop, he has told you something: strong hands — top pair with a good kicker, overpairs, sets — almost always c-bet, so the hands that check back are mostly medium pairs, ace-high and missed draws. On the turn you can bet for value with hands that beat that capped range and need protection, and bluff on cards that look scary for it. Waiting for him to bet again lets his medium hands see a free river.
The probe is a turn play, not a flop lead. On the flop the right default against a raiser is to check and let him c-bet, then raise with the hands that want to; leading takes away the bet your check-raise wants to attack. Probes also need a card or a hand that justifies them: a scare card for his capped range, a pair that is now ahead of nearly everything that stays, or a draw with fold equity on top. Even an OESD on a turn after a check-check flop is a bet: 8 outs plus the folds of a range that could not bet the flop.
Worked example
FAQ
What is the difference between a probe bet and a donk bet?
A donk bet leads into the preflop raiser before he has had a chance to c-bet. A probe comes after he checked back the flop: his check has capped his range, so betting into him on the turn is much safer.
What size should a probe bet be?
This trainer’s default is about two-thirds of the pot when the turn is a scare card or improves you. The size must work for both your value hands and your bluffs, because they bet together.