This is closely related to blocking out of date pull requests, which has more details of why you would want to smooth the workflow around pull requests.

This post-receive hook will fire if the developer is pushing any changes on a branch, for which a pull request exists with a corresponding from branch. It warns developers if their topic branch is lagging behind their target branch - showing how many commits behind they are, and the date of the first commit on the target branch which is not reachable from their topic branch.

The output will be similar to:

remote: View pull request for topic => master:
remote:   http://localhost:7990/bitbucket/projects/TEST/repos/test/pull-requests/2
remote: The branch 'topic' is 2 commit(s) behind master.
remote:   The first commit on the target that you're missing was at Fri Jun 05 14:12:53 BST 2015 (server's timezone).
remote:   Please consider rebasing.

This gives the developer an idea of how far they are lagging their target branch.

Additionally, should there be conflicts with the target branch, they will also see:

remote: In addition the following files have conflicts:
remote:   include/ap_compat.h
remote:   include/mod_request.h

The intention is to let developers know about merge conflicts as soon as possible, as the earlier you resolve them the easier it is, plus it keeps the feature branch "deliverable".