Under the source-type bar on Sources, one line compares two groups of answers: Mentioned in 72% of answers that cite your site, 31% of those that don't (the figures are an example). Two small bars sit above it. Hover for the counts.
How it is worked out
Each answer counts once, however many times it links to your site. The first group is answers that link to your own website. The second is every answer with no link to it. The share is how many of each group name your brand.
The figure is hidden when either group has fewer than 30 answers, because with fewer one answer moves the percentage by more than 3 points.
This compares two groups. It does not show that the link caused the mention: answers that cite your site are often the ones where you already fit the question well.
Where to find it
Sources, under the source-type bar.