This kind of annoys me a little. Basically I'll be working on a feature branch and I've submitted a diff, but I'm still waiting for it to be accepted. I want to start the next piece of work, so I arc feature off the feature branch (thus it's master -> first_feature_being_reviewed -> second_feature).
When the review is accepted and landed, I then have to do git rebase --onto to move the second feature branch on top of master. Ideally arc land would detect any derived branches and rebase them automatically, but I'm guessing this could be quite difficult to do properly (especially if there's 3 or 4 feature branches).