For background, see Genetic Genealogy for Jewish Ancestry and Overcoming Endogamy in DNA (YouTube) by GeneaVlogger.
If you take a DNA test to find relatives, and you’re from an endogamous population (such as being Ashkenazi), you need to look at your matches with the longest shared stretch of DNA. That’s easy enough to do on MyHeritage or FamilyTreeDNA, but not possible in Ancestry or 23andMe as they do not display or let you sort by longest segment/block.
It doesn’t actually sort your matches. What it does do is calculate the ratio (total/segments) for each match displayed on the current page, hide matches below a certain threshold, and add the ratio number to each match. On Ancestry, which uses infinite scroll, you’ll want to scroll the page to reveal lots of matches before you click the bookmarklet. On 23andMe, you’ll want to paginate and rerun the bookmarklet on each page.
The calculation uses the total amount of shared DNA in centimorgans, however since 23andMe displays the total as a percentage, it firsts multiplies the percentage by 75 to get a fairly accurate centimorgan number. Some websites claim that the multiplier should be 68, but I believe this number was calculated for FamilyTreeDNA, not 23andMe.