It's two factors, both the match on an image hash and an unknown threshold of matches at which point the data gets sent up. If the threshold is not met then not

It's two factors, both the match on an image hash and an unknown threshold of matches at which point the data gets sent up. If the threshold is not met then nothing gets notified (even if there is a match). Arguably this is why this approach is better for privacy. Cloud matches would not be able to have this extra threshold (in addition to this model allowing e2ee on the cloud in the future). I'd also like to know more about the specifics here, my guess is that threshold value is pretty high (their 'one in a trillion' comment not withstanding). It's probably targeting large CSAM dumps of matches which would not get flagged by different images.