Blocker bet

The classic spot is the river, out of position, with a hand too good to fold and not good enough to call a big bet with comfortably. Checking invites a bet of half the pot or more; betting a quarter of it often ends the hand at a price the player has chosen rather than one the opponent has chosen. The bet blocks the bigger bet - hence the name.

The weakness is that it is transparent to a thinking opponent, and a strong one punishes it in two ways: raising with hands that have real value, and raising as a bluff precisely because the small bet advertises a capped range. Against that opponent, checking and defending correctly is usually better.

It is also worth separating the move from a thin value bet, which is made to be called by worse hands. A blocker bet is content with a fold and expects to show down; the sizing follows from that intention, not from the hand's strength.

Solver work tends to use small river sizings for a different reason - as part of a range that includes strong hands and bluffs at the same size - which is why a bet that looks like a blocker bet in a solver output is not necessarily playing the same role.