Positions come past you in a stream. Some of them are checkmate — Black to move, no escape. Every other position is a decoy: usually Black in check but able to get out, sometimes stalemate, which looks like mate and isn't. Press the moment you see a real mate.
The thing being measured isn't how fast you spot one mate. It's what happens right after. When you catch a target, your attention is briefly tied up processing it, and for roughly the next 200 to 500 milliseconds you are close to blind to a second target. Psychologists call it the attentional blink. It is not an eye problem — your eyes are open and pointed at the board. The information arrives and does not get through.
Two mates are hidden in each stream, at a controlled distance apart. The report shows how often you caught the second one as a function of the gap. That curve is your blink window.
Over the board this is the shape of a specific, familiar disaster: you see the tactic, you feel the lift of having seen it, and in that half second his reply goes straight past you. Same mechanism, slowed down and given a number.