I hadn't noticed that you could do the equivalent of HubMed's "Rank Relations" in PubMed as well: select multiple items, then choose "Related Items" from the drop-down menu. It shows items that are closely related to any/all(?) of the original selection. I'm not sure what algorithm it uses: here's an example in PubMed, and the result from the same initial selection in HubMed. The first two items are the seeds.

In PubMed, you can also do the same thing for "Cited Articles", i.e. show articles that are highly cited by items in the original selection (limited to reference data from articles deposited in PubMed Central, presumably), which is useful.