Xman, the pocket is between the pleat folds , and the normal hair canvas isn't affected, but as I said I generally add an extra layer into the pleats that are used. I still stitch thorugh the pleats on the inside so the internal strength isn't altered really.The actual pocket is an extra "bag" sewn in . I put it in in the bottom half of the fell area, but make sure that I have at least an inch or so of stitching to complete the bottom of the fell.
It is only a small pocket, and it doesn't suit every kilt. If I can find, or make any photos of an example I'll try to post them, seeing it would making the explanation so much easier!