The process of back cutting, or sometimes called back-bevel, quite simply is cutting a little more off the back side of the cut so that there is less material towards the rear edge of the trim piece. By removing slightly more material from the back side of the trim piece (not too much) when making the cut you ensure that the front side extends out such that only the front edges of the two adjoining pieces of trim actually touch. This enables a thin edge on the front side of the trim, which is much easier to get a tightly joined seam.
There are a couple of ways to accomplish this, both are quite easy and can be done by anyone. If you have a compound miter saw, change the angle of the saw to 1 or 2 degrees off 90. This gives a slight back cut on the trim. Remember you want to still cut the trim on a 45 degree angle your changing the bevel of the saw in relationship to the trim. The next method simply involves setting a thin strip of material under your trim before you cut it. It should be about 1/8 to 1/4 inch thick. Using this method you won't need to keep moving and adjusting the angle of your miter saw. This method can also be used with a out a compound miter saw.
Yes, there will be a slight gap towards the back, however you can minimize the gap by limiting the back cut to just over-cutting one or two degrees. This will ensure that the front edges of the adjoining trim pieces fit together perfectly at their outer edges. Any gaps are usually not visible once the trim is installed.
Watch our installing door trim video for great tips and see how to accomplish back cutting with a miter saw.