Called "Team Sneaking," the mode is similar to "Sneaking" in that one team – not just Snake as in the latter mode's case – is invisible until detected and must hide from the opposing team until the Caution expires. The sneaking team also has an objective to complete inside their opponents' base.
Today's update also adds a "Headshots Only" option for game hosts. As the name implies, games with this modifier will only reward points for headshots. (Kills from shots to the body will actually subtract points from players' scores.) The update is live now and free of charge. Just fire up MGS4, head into MGO, and you're good to "Go! Go! Go!"