Fowling a barrel is an old shooters trick. The point is for consistancy shot to shot, you want all things the same. So if your first shot is on a sparkling clean oil free barrel, and your next shot is on a powder crusted, swabbed barrel, that is an inconsistancy in many people's mind. Since the condition of the bore has changed.
Now consider you fowl the barrel and swab it. So you now have a powder crusted, swabbed barrel and you shoot. Your next shot after you swab is again, a swabbed powder crusted barrel, you have consistancy somewhat. So if consistancy makes a big difference in your rifle, then you should shoot a better group this time, if you count your first shot.
What you need to do is start on a clean barrel and shoot at 100 yards. Now swab the barrel and shoot again. Swab the barrel and shoot again. Did the first shot hit on a different area then the other two? If not, then you have no need to swab the barrel. If the first shot was only a half inch or so off, again no need to swab the barrel. BUT I have a rifle that on a clean barrel will shoot an inch and a half high and a little left. After that it will shoot out the bulls eye. So if I want that first shot to be exactly where my scope is set, then I need to fowl, or sight in on a perfect clean barrel.
When I fowl a barrel I like to dump 40 grains of the powder I am going to use, then push a wad down on the charge, or a patch. Cap it, aim somewhere safe, and fire it. Now swab like normal. You now have a fowled barrel.