They are made for drop-throughs. If you look at the pictures, you should be able to work it out, but…
With a drop-through longboard deck, until you lean it, the deck hangs from the screws the hold the truck on. So, with the longboard at the top picture, if I someone unscrews the trucks, while I stand on it, the deck will fall down against the hanger.
I could take the spacers out, assemble it like the picture, put the nuts on, with just a few threads, and the deck will just hang from the screws, and leave a big opening around the truck.
So, the thicker the spacer, the lower the deck and rider, as long as the truck doesn’t hit the deck when leaning. Fastened with longer than stock screws, now the deck is almost a half and inch lower than it was.
Are you ready?
The spacers help constrain (bind) one of the six degrees of freedom. In this case, using the roll, pitch, yaw (configuration space), they help constrain the roll axis of the board in relationship to truck, so that…it may turn, by the means of a virtual hinge axis which can be drawn from the center pivot cup to a point in the center point of the hangar bushing hole.
And once you hit a bump or push the board forward (if you have no motor for it), or do anything to compress the bushings–or even the wheels, or even bend the board, then, well, it gets complicated.
Anyway, I think they will work better than a stack of washers or nuts, and I wanted to share them with people.