Defensive Replacement
A better glove comes in late to protect a lead.
How it actually works. Typically it is a corner outfielder or a first baseman coming out in the eighth with a lead, replaced by somebody whose bat would not play in a starting lineup. It costs the club whatever that hitter would have done in his last plate appearance, which with a lead in the eighth is often nothing at all.
Why it is done, and when it is wrong. The logic is a straightforward accounting of what is left in the game. With six outs to record and a two run lead, the number of balls that will be hit toward left field is larger than the number of plate appearances that spot will get. It is the same principle as the no doubles alignment, applied to personnel instead of position.
What people get wrong about it. It goes wrong when the game is tied later than expected and the manager is left with his worst hitter due up in a spot that decides it. Every defensive replacement is a bet that the lead holds.
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