:bbGitHub step is exactly like the :bbGit step, except that it will ignore the revision sent by :bbGitHub change hook, and rather take the branch if the branch ends with /merge.
This allows to test github pull requests merged directly into the mainline.
GitHub indeed provides refs/origin/pull/NNN/merge on top of refs/origin/pull/NNN/head which is a magic ref that always create a merge commit to the latest version of the mainline (i.e. the target branch for the pull request).
The revision in the GitHub event points to /head is important for the GitHub reporter as this is the revision that will be tagged with a CI status when the build is finished.
If you want to use :bbTrigger to create sub tests and want to have the GitHub reporter still update the original revision, make sure you set updateSourceStamp=False in the :bbTrigger configuration.