In a wing shape moving along level with the horizon, the force is roughly upwards.
I'm pretty sure the forces acting underwater are similar, or at least behave similarly, to those that work in air.
The force acting on a lure is also roughly upwards.
|those diagonal purple wind lines are from a different diagram|
It also has a bib. A bib doesn't act as a wing, but rather, acts more like a rudder. The bib adds length to the lure without adding lift at the front of the lure. The bib also encourages the lure to follow the direction to which the bib points, because it makes it too hard for the lure to swim directly towards the fishing rod. Point the bib down, and the lure will go down. The bib is set so that it presents as a flat plate resisting being pulled along. Picture pushing a flat dinner plate through the water. If you turn it edge on, its much easier to push. If you drop the plate flat into water, it wont just sink straight down, it will try its hardest to get some sideways to add to it's downwards. In much the same way, the bib wants to move in any direction other than flat up or flat down. Because the lure is being pulled along from roughly the front, the bib is limited in just how stubborn it can be. It negotiates a compromise and moves generally forwards, a bit down, and has a go at moving a bit to each side. The attached line gives it a bias to forwards.
A dropped dinner plate might also go to one side, spill a bit of pressure, then rock back to the other side. This may repeat so the path of a dropped plate may well be a zig zag all the way to the bottom. I generally encourage experimentation, but if you must drop plates into the bath, I suggest waiting until you find yourself bathing at someone else's place, and use their plates. Interestingly, and not without an incredible amount of forward planing on my part, this dinner plate zig zag might also go some way to explaining how a bibbed lure gets it's swimming action.
Some kind of vague approximation of that effect is depicted here by green arrows pivoting around a drawing of a bow, tied in imaginary string.
A long flat bib encourages the lure to swim to the bottom, a short, sharply downwards angled bib makes for a shallow diving lure.
Exactly why a short, steeply angled bib makes for a shallow diving lure remains a mystery to me, so for the time being, will remain unexplored.