Tension-Release Slash Notches
If you are sewing an inward-facing curved seam, you may find that the finished seam does not lay as smooth and flat, retaining some unwanted tension. In this case, a few simple slash notches can be used to release tension, especially within abruptly curved edges. When applying slash notches, you should use the least amount of notching necessary to release tension and allow the seam to lay flat. The idea is for the seam allowance layers to incur the least amount of damage form this process as not only will this preserve the longevity of the seam, it will also give it strength and durability during wear wash cycles.
To apply the slash notches, simply clip the seam allowance horizontally from the edge inward, starting at fabric's raw edges and ending at about 1/8" away from the stitch. The idea is to slash near the stitch but not so close that it actually weakens the finished seam.