Hi new syntax is not a secret, but I preferred to not show the new syntax in full in the help dialog.
The reason I changed it is that too many users did thing like "DCS_updater update ka50" thus breaking their installs. Hmm, I've just realized that they probably edited their autoupdate.cfg files and the new syntax won't help. The updater does check if the branch name is a valid one.
Anyway, the new syntax is [version][@branch], like
DCS_updater.exe update @release
DCS_updater.exe update @openbeta
DCS_updater.exe update @openalpha
or
DCS_updater.exe update 1.5.1@openbeta