Part: Part Seven - Betting on Fourth and Fifth Street

A blocker bluff

Turn
Heads-upPot 3,000 (15 BB)Q842BTNAK 25,000 (125 BB)YOUace-high with the nut-flush blockerBB 25,000 (125 BB)Big blindchecksD

You raised pre with A♠K♦ and c-bet the Q♠ 8♠ 4♣ flop; the big blind called. Turn 2♥ - you have only ace-high, but you hold the A♠ (the nut-flush blocker). He checks.

Ace-high, no pair, but you hold the A♠ on a two-spade board. Best on the turn?

WhyCheck behind. With no pair, betting again is a thin bluff into a caller; checking controls the pot and - crucially - keeps your A♠ live to bluff a spade river credibly. Your blocker is most valuable as a river weapon.
What happensYou check behind.  Pot stays 3,000 (15 BB).
River
Heads-upPot 3,000 (15 BB)Q8427BTNAK 25,000 (125 BB)YOUmissed, but holding the A♠BB 25,000 (125 BB)Big blindchecksD

River 7♠ - a third spade completes a possible flush. He checks. You missed everything, but you hold the A♠.

The flush completes, he checks, and you hold the nut-flush blocker. Best?

WhyBluff. Because you hold the A♠, your opponent almost never has the nut flush, and his other hands (a pair, a busted draw) struggle to call a big bet on a three-flush board. Betting credibly represents the flush you're blocking - the perfect spot for a river bluff.
What happensYou bet 2,400; he folds his pair.  The blocker bluff gets through.
You preserved your ace of spades by checking the turn, then fired the river when the flush completed - holding the blocker meant your opponent rarely had the hand you were representing. Blockers turn missed draws into the best bluffs.

Bluff rivers where you block the nuts you're representing - holding a key card means the opponent rarely has the hand that can call.