Part: Part Nine - Inflection Points: M, Q & the Zones

Calling ranges are tighter

Pre-flop
Blinds 400 / 800 (100 ante)Pot 11,100 (13.9 BB)Your M = 4 • Red zoneBBK♠10♠● 8,400 (10.5 BB)YOUto actMP● 9,000 (11.2 BB)Short stackall-in 9,000 (11.2 BB)SBfoldsD

Red zone (M 4). An early-position short stack open-jams about 11 BB, and you hold K♠T♠ in the big blind.

An early-position short stack jams; you hold K♠T♠ in the BB. Best?

WhyFirst-in you'd happily jam K-T suited, because folds are half the profit. Facing a shove that half is gone - you win only when your cards beat his - and an early-position jam is weighted toward big aces and pairs that dominate you. K-T suited is flipping at best and crushed too often, so the same hand that's a fine push is an easy fold here.
What happensYou release it, and the gap comes into focus: what you push and what you call are two different ranges, not one.
Never conflate your push range with your call range. Shoving buys you fold equity; calling buys only your raw equity against a range, so the hands you can profitably call with are far fewer.

Call a shove with a tighter range than you'd push - calling has no fold equity, so you need a hand that actually beats the jamming range, not just one you'd open with.