In bitbucket/git, what does "xx commits behind proto. Sync Now" do?
so
Sync Nowwould merge the all the new commits inprotointo my current feature branch?
It is the "Sync now" feature presented in this 2018 thread (initially for SVN, and for complaining that feature was not visible enough in the new BitBucket Cloud web UI).
In the case of "x commits behind" and Git, it would actually simply move the HEAD of your main branch (master) to the latest of the original repository.
Your feature branch should be unchanged (you can rebase it on top of master after sync if you want).