What does Ciaffone mean by overlay?

[BobboFitos]

mason solid post. but, and yeah i'm being nitpicky:

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Say a site offers a $100 guarantee prize pool on a $1+0 buyin. If 100 people enter, your share of the prize pool is 1%, exactly what you contributed. If only 50 people enter, you have a 2% share of the prize pool but only contributed 1%, thus you have a 1% overlay.

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this is incorrect