For disposables, if you wet while standing only, sheer physics discourages a complete wetting of the back part of the diaper (and the front part, for that matter). Liquid will go into the low part of the diaper between your legs and after the SAP has absorbed it, that is where it will for the most part stay. This is why the superabsorbent core of the diaper (and most of the absorbency, leak prevention, and acquisition layer length) is concentrated between the legs.
Now, if the SAP in the core is completely saturated, then the pulp retains the wetting, and the pulp can wick some distance (like a cloth diaper can). Then, naturally the diaper will start to wick to the front and back, but only so far as physics and the materials allow. In normal use I see a lot of wicking to the back, but less to the front, which probably has to do with press-out (which I will mention in a second).
The way I understand it, the extra padding that doesn't always get wet in front and back is there to help the diaper keep you dry in different postures. The front is for when you're on your belly and also for male wearers, where the "garden hose" providing all of their wettings can be positioned in any direction, worst case straight up. The rear of the diaper is for laying down on your back and sitting. When sitting or laying your butt puts a lot of pressure on the pulp and SAP, which forces liquid out, so it needs somewhere to go. Further, when laying down and wetting, the diaper needs more material to slow down the flow and to catch liquid that isn't immediately grabbed by the rest of the diaper. You can sometimes physically feel this happening when you wet while laying down.
Long story short, wet a couple of times while you're on your back, and you will get a soggy bottom in no time. You can also saturate the core and then sit in it (on a hard surface, preferably). Just watch for press-out leaks!
once you find the right place to tape it (depending where the tip of your penis lands) half the flow is directed directly to the back of the diaper ! 100 % use every time. Works even better when sitting.
