This headset tensioner is for a 1" fork.
Allows you to place a load on your threadless headset, when a star nut application will not work. This works great for "Stiffy" type steering applications. Basically, if a pivot type mechanism is not used to clamp and hold the fork steerer, this tensioner coupled with a stem clamp will do the same job. The tensioner is basically a big bolt and nut. With a tightened stem clamp, you loosen the bolt and nut. They back up against the stem clamp and against the headset thus tightening the bearings.
Please note with this in place, you cannot raise and lower the stem. Height changes are accomplished with stem shims.