This extension replaces BitBucket's default pull request commit message with a more useful message, especially if you squash PRs.
If your development workflow involves creating a pull request for every change, and you squash your PRs with the default commit message when you merge them, then your commit history has a bunch of entries that all look like "Merged in my-terse-branch-name (pull request #123)". These aren't especially helpful commit messages and "Merged in" just becomes noise. This extension will use the title of your PR as the commit message instead which should lead to a much more useful commit history. The bulleted list of individual commit messages and approvals will still appear in the body of the commit message.
To tell Atlassian that this should be the default behavior, go to the following URL and vote for the issue.