Here's three ways you can carry IWB:
1) A folder with a clip can be clipped to your waistband, knife inside, your belt over the clip, hiding it.
2) A fixed blade with a clip on the sheath can be worn the same way but with the handle sticking up it isn't as hidden. One way to compensate for this is to wear it almost horizontally.
3) A fixed blade can also be worn with a loop attached to the sheath with the loop slipped around your belt. When you withdraw the knife, the sheath begins to come up with it, but the loop stops it, and the knife pops out.
Some IWB sheaths come with a J-hook, a J-shaped piece of plastic that slips under the belt to hold the sheath from coming up, but the rig can be removed without unfastening the belt.
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I find IWB is especially effective just to the left of the belt buckle for a crossdraw for small fixed blades, or just in front of my right hip for folders. It can also work SOB which is Small Of the Back, which tends to be even less noticeable but harder to access.