What is "submarining?" The "old submarine special" sounds like something at a sandwich shop, haha...
The sandwich shop version is much, much more pleasant, I assure you.
(From here on, this post is far too long and contains way more information than you asked for. But here goes...)
In the case of seat belts, and particularly racing harnesses, it refers to the situation where following an impact, the body first starts to lurch forward but the upper body is held in place by the belt. Inertia makes the body still attempt to move forward, resulting in a the legs and hips moving forward sliding the lower torso underneath the lap belt - submarining. This is extremely unpleasant, as the lap belt then forces itself inwards and upwards into the squishy parts, resulting in ruptured spleens, kidneys, and other nastiness. This is why the 5th (or 6th) belt that comes up through the crotch in a 5 point harness is called the Anti Submarine Strap.
With a normal street 3 point belt like yours, this isn't an issue. Having the belt over only one shoulder allows the torso to move forward and twist slightly, positioning the pelvis such that submarining doesn't occur.
The concern raised here regarding the adjuster is that if the lap belt is pulled upwards too much, it may be high enough that the "pelvic twist" is no longer sufficient to prevent submarining.
Just watch how you adjust the belt. Keep it nice and low across the lap and you'll be fine.
