In a binary market the YES and NO shares together are guaranteed to pay exactly $1.00, so their prices should sum to $1.00. When the two sides sum to more than $1.00 you can sell both and keep the excess regardless of the outcome; when they sum to less, you can buy both.
The profit on a complement trade is locked in the moment both legs fill — the risk is not directional, it is execution and time. You need both fills, and you need to fund the position until the market resolves.
In practice the sum rarely deviates far on liquid markets, and when it does the window is short. That is why this is a latency-and-scanning strategy rather than a patient one.
Treating a deviation from $1.00 as free money. The profit is real but it is not instant: both legs have to fill, and the capital is locked until the market resolves, so the annualised return can be far worse than the headline gap suggests.
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