Bluff catcher
A hand that only wins if your opponent is bluffing: it loses to all their value. Calling or folding it is the purest river decision there is.
A bluff catcher is a medium-strength hand on the river with one very specific property: it loses to every hand your opponent bets for value, and beats all of their bluffs. Q♠Q♦ on an A♣8♦5♠3♥2♣ board facing a big bet is the textbook example: any ace has you beat, any bluff doesn’t.
The price of catching
The decision is pure arithmetic. Against a pot-sized bet you’re getting 2:1: your call needs to win 33% of the time. The question is no longer “am I ahead?” — almost certainly not against the value — but “are they bluffing more than one time in three?”. Against a half-pot bet, more than 25% is enough.
Why you can’t fold them all
If you fold every bluff catcher, any alert opponent prints money betting the river with anything. The theory (GTO) says that against a pot-sized bet you must defend half your range to stay unexploitable — which turns these hands into mandatory calls at some frequency, even though every single one feels terrible.
In practice, you pick the frequency with information: against the opponent who never bluffs, the disciplined fold makes money; against the aggressive one, your sad pair of queens pays it off. And blockers tilt specific hands: blocking their bluffs argues for a fold, blocking their value argues for a call. The trainer’s bluff-catching drill deals you these rivers one after another, with the solution computed against the real betting range.