The bolt doesn't tension as such, it just locks (when you tighten) the tensioner mechanism at the place it's set to.
Unlocking the bolt lets the internal mechanism move to it's "correct" position via it's own spring, you then lock at that position for all subsequent running until the next time you want to alter it.
In effect, you just nip the main bolt onto the internal tensioner, then secure that position with the nut to now prevent it backing out.