This is a very simple prop balancer which is easy to use. Fit the prop hub in between the two tapered cones, then hold the two pointed ends of the shaft lightly between thumb and forefinger, let the prop settle and you should quite easily see which blade is the heavier.
For boat props try this:
Get a flat piece of wood and get 2 new Stanley knife blades and glue them so they are standing vertically to form 2 knife like edges. Mount the simple prop balance and prop so that the shafts at both ends are resting on the knife edges and hey! Presto!. This is a very low friction set up
1 spin of 2 is bad quality, the shaft twisted and the extremity are flat no conical.
After some work on it, that become correctly for the shaft, but for the Spin I replacing it with a nut and washer.
I hope Hobbyking have to check the item in stock because my DIY balancer with threaded rod is better if no work is done in this part.
That cost the little price maybe...