After finding myself baffled attempting to bring a feature branch up to date with development and failing, I've come to realize that the left-pane context menu item labeled “rebase current changes onto $somebranch” actually does the opposite of what its name suggests:
it rebases the current branch to the state of $somebranch;
In other words it rebases $somebranch onto (or into) the current branch, not the other way around. (Right?)
The preposition “onto” in the current text is misleading; it implies that the object of the sentence ($somebranch in my example) will receive the changes.
In fact, it is the opposite that will occur.
The absence of the current branch's name adds to the confusion.
A re-wording that improves the sentence structure and includes the name of the affected branch would afford a huge win for clarity.
For example:
rebase $currentbranch to head of $somebranch
rebase $somebranch onto $currentbranch