James I'm sort of winging here but the force generated by the tensioner just has to be enough to take up the slack. When they aren't pumped up with oil you can easily compress them against your bench with one hand.
With a static engine if you turn the crank backwards it would be the valve springs against the tensioner spring (plus quite a bit of chain drag). The cams would hold the IMS still until the slack side was pulled tight.
By comparison, my Triumph motorcycles are inline triples - sort of like half a "real" motor.
One cam chain, one tensioner which is a long but quite beefy spring. 9750 rpm as much as i like (and i do). Off topic but when you take a cane to the triumph triple it sounds similar to a nasty flat six. And stone axe reliable i might add.