Part: Part Five - Betting Before the Flop

A small pair, short-stacked

Pre-flop
Blinds 100 / 200Pot 900 (4.5 BB)CO6♠6♦● 3,000 (15 BB)YOU15 BB stackUTG● 9,000 (45 BB)Openerraises 500 (2.5 BB)SB● 9,000 (45 BB)posts 100 (0.5 BB)BB● 9,000 (45 BB)posts 200 (1 BB)BTN● 9,000 (45 BB)BTND

You have a 15 BB stack. A player opens to 500 from under the gun and it folds to you on the cutoff with 6♠6♦.

15 BB deep, an opener makes it 500 (2.5 BB); you hold 6♠6♦ on the cutoff. Best?

WhyThree-bet shove. Short-stacked, you lack the implied odds to set-mine - calling 500 with only 15 BB behind can't pay you off when you hit. A shove takes the pot's dead money plus the blinds via fold equity, and you're rarely in terrible shape when called. Set-mining is a deep-stack play.
What happensYou shove 15 BB; the opener folds a hand like A-J.  You pick up a useful pot.
The same pocket sixes you'd set-mine 150 BB deep become a shove at 15 BB - without implied odds, you take your equity plus fold equity now. Stack depth changes the whole plan.

Set-mining needs deep stacks for the implied-odds payoff; short-stacked, small pairs are shove-or-fold hands, not call-and-pray hands.