How to move some changeset to a new branch in mercurial

I want to move a changeset from one branch to another. Basically, I currently have:

A -> B -> C -> D # default branch

And I want:

A # default branch
 \-> B -> C -> D # some_new_branch

Where some_new_branch does not exist yet. I am used to git, so I guess there is a simple "mercurial" way I am missing.