SPR

Take a 1/2 game, 200 effective. Button opens to 8, big blind calls, and the flop pot is 17 — 8 from each player plus the folded small blind's dead dollar. That leaves 192 behind, so SPR sits around 11. Room to maneuver. Same seat, same open, but this time the big blind three-bets to 30 with AK and the button flats JJ. Now the pot is 61, stacks behind are 170, and SPR is near 2.8. At that ratio a pot-size bet and a raise puts both players all in, so an overpair or top pair is a stack-off hand the moment chips start moving. Nothing left to fold to.

SPR tells you how many bets are left to fight over before someone's stack runs dry. Low SPR strips the hand down to raw strength — nobody folds a big pair with only a couple of pot-size bets behind it. High SPR pays off patience and disguise: suited connectors, small pairs hunting a set, and draws all gain value, because there's enough behind to get paid when you hit and enough room to fold cheap when you miss.

The common leak is treating every strong hand the same regardless of SPR. Stacking off an overpair at SPR 11 because it "never folds preflop" ignores that deep stacks let opponents keep peeling with hands that pass you by the river — sets, two pair, straights that only turn up on later streets. Check the ratio before you commit chips, not after they're already in the middle.