Thinning shears have notches cut into the blade. Some on one blade, some on both blades. Some are very narrow, some are very wide apart. When you cut the hair with them, only some of the hair gets cut and some does not. That is how you "thin" the hair. I think you already can see this working well — and really making a mess of it when done incorrectly!

It takes a lot of experience to know how to use any of these tools to give you the effect you want. It will take a chapter in a book on cutting to explain how to do it correctly. But my suggestion is that you get the shears that cut the least amount of hair at a time. That way, you have more control over how much is removed. If you remove half the hair in one chop, that is too much, and you will end up with a short clump that sticks up. If you remove only a little at a time, you will be able to stop when it is enough. You can remove more, you cannot ever put any back. Basic rule of cutting hair.
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