Part: Part Twelve - Heads-Up

Three-betting heads-up

Pre-flop
Heads-up • Blinds 400 / 800Pot 2,800 (3.5 BB)BBA♣9♦● 20,000 (25 BB)YOUto actBTN● 20,000 (25 BB)Opponentraises 2,000 (2.5 BB)D

Heads-up. The button raises to 2,000 and you're in the big blind with A♣9♦.

Heads-up, button raises; you hold A♣9♦ in the BB. Best?

WhyStack A-9 offsuit against the range a button opens heads-up and you are ahead of the average hand in it - that is what makes this value rather than a bluff. Re-raising grows the pot while you hold the better hand and grabs the lead; calling invites him to outdraw you in position, and folding is far tighter than the spot warrants.
What happensYou three-bet and either take it down now or play a bigger pot as the favorite. Heads-up, aces this strong get raised, not called.
A-9 offsuit is comfortably ahead of a wide button-opening range, which makes this a value three-bet. Flatting lets him realize equity in position; folding an ace this strong heads-up is far too tight.

Heads-up three-bets are often for value, not just pressure - a strong ace is genuinely ahead of the range a button opens, so raising it builds the pot while you're in front.