Yeah, you know, for the money they spend on marketing and the cost of their packs they sure do a craptastic job of explaining.
They sell the Deuter Race Air at REI, if there's one near you you should be able to check one out in person -
Deuter Race EXP Air Bike Hydration Pack - 100 fl. oz. - Free Shipping at REI.com
That style of design (I've seen others use it) is the only thing that really keeps your back dry (note: keeps 95% of your back dry, the part where the non-mesh part of the pack contacts your back can still get sweaty, but it's a smallish area at the top and bottom, rather than your whole back).
The part that's in contact with your back is a mesh panel that breaths. Behind that is a curved plastic panel that makes up the back of the pack. That plastic piece makes sure there's always space between the mesh part on the back and the plastic panel, so air freely flows through the entire area.
It's the only design I've seen that actually keeps your back dry. All the other designs are more like "slightly less sweaty than otherwise".
It's been great for me on hikes, for mountain biking. The drawback of the design is that the hard plastic piece keeps the back from bending, so it's not great if you're riding a road bike that requires to you hunch over the handlebars a lot.
You can get racks that go on a road bike without rack braze-ons. Here's a pic of the Bontrager Lightweight rack on a full carbon Trek Domane with no rack mounts -
Axiom and Tubus also make similar racks that mount without rack mounts if you want something cheaper (Axiom) or more sophisticated (Tubus).
And there's other bag options as well - oversized seat bags, handlebar bags, top tube bags, frame bags, etc.