The free dating site figures out two people's compatibility by how they answer specific questions: The more similarly answered questions, the higher the chance of a good match.
After striking out he began to use a super computer he had access to through his grad school to analyze OKCupid's question data.
He was able to separate the woman around him into seven groups, figure out which had the highest chance of compatibility with him, and which questions were most important to the women from that group.
Once he knew the key questions, he answered them honestly and became the top match for 30,000 women and began receiving a huge increase in unsolicited messages. Though popular mathematically, McKinlay found he had to deal with woman's expectations that the two were "perfect matches."
He combated this social expectation by going on "efficient and depersonalized dates" one after the other until he finally found his true perfect match, on date #88.
Here's a video of McKinlay explaining his dating hack: