MDF (Minimum Defense Frequency) is calculated as 1 minus alpha, where alpha is the fraction of the pot the opponent risks with their bet. For example, against a full pot bet (alpha = 50%), MDF is 50%, meaning the defender must call or reraise at least 50% of the time to prevent immediate profitable bluffs. MDF is a valuable approximation tool for simple spots but does not capture the complexity of ranges with varying equities. In multiway situations or with multiple streets remaining, pure MDF may understate the defense required for true equilibrium.