Oversimplified but I'll give it a try. Because smokeless powders in the small amount necessary to produce BP equivalent pressures don't come near to filling large black powder cases, some shooters feel it is required to put a wad down on the powder to keep it at the primer end of the case or to fill the rest of the case with various and sundry fillers. You may recall that certain antitank projectiles have a small "shaped" charge which is not sufficient to blow anything up but will by focusing the shock wave of the explosion cut a hole right through the armor. The shock wave produced by powder levelly retained by a wad propagates up the case until the wad contacts the base of the bullet where the shockwave is deflected sideways against the chamber wall forming a ring in a chamber. This may happen on the first shot or increase through several shots until the case starts to stick in the chamber. If you'll look up Charlie Dell and chamber ringing you will find where he was able to do it even without a wad by shooting straight up. The key factor is that the powder had a level surface at a right angle to the axis of the case and had free length to propagate itself until encountering the bullet. Some fillers allow the propagation to happen and form a wad from the pressure. Using a wad but keeping it a small distance (say 1/8 inch) off the powder so that the powder's surface can be not at right angles to the bore is OK for some folks. Best to just use a powder that is easily ignited and just tilt the case up a bit when loading to let it mostly be towards the rear.
