You put runners through the holes on the business end of it. The runners have female ends, and a balance wheel mounted on the staff is mounted between the runners. A flat piece projects out into the space surrounded by the arms and runners, being fixed to the frame (probably where the hole is near the handle) in a manner to fit closely and lay against the wheel. This piece acts like an indicator, in that the wheel is rotated and the any inconsistancies in truth are guaged by looking at the gap between the indicator and the side of the wheel while turning the wheel. Any untruth of the round can be seen by looking down from above. Then you use a wrench or tool to true the wheel.
That frame is a good pattern to have, and runners are no real hard feat to make in the lathe, and the indicator can be cut from flat stock and polished up. I made three sets for truing different sized wheels, and three more sets for finding the total length of a staff.
The runners for determining staff length have flat pads left hard, and one is half again shorter than the other, but will meet comfortably and have no gap between the pads. The short runner is pushed in all the way and fixed in place, and the longer one backed out till the tool will fit over the movement in question, from wich the cap jewels of the balance cock and foot have been removed, and the cock mounted to the plate. The flat pad of the runners ought to rest on the seat for the cap jewels. Tighten the longer runner in place, then unlock the short runner and remove from the movement. You have only to push the short runner back in all the way and secure it, and the distance between runners will be the overall length of the staff.
That'd be a pretty nice tool to have all in one piece. If you lay it across your lathe bed with the business end facing you, you can poise a balance wheel with it, too.