how do they work? what yoyo would you use it for. Pics would help.
Shims look like this (the red piece):
They work in most YoYoJam plastics (Kickside, Lyn Fury, etc.) and hybrids (Dark Magic, Speeder, etc.), as well as in most of the SPYY D-bearing yoyos and others (though the D-bearing SPYYs require a different size shim than that you use in YYJs).
See also: YoYoWiki.org/Shims