This one breaks the images in their database into blocks, hashes those blocks separately, then checks your images by matching those hashes against same sized block hashes. It needs only a certain number to claim a positive match, and in theory it should be manually checked.

However pedos tend to have 10000s of images and they aren't going to all be manually checked so the process is going to be trusted rather than proven for each case. This is risky as it's only matching some blocks rather than all the blocks for the whole image and can be defeated by simple filters or changing enough of the blocks to make the test meaningless.