He means the roll-top closure at the opposite end to the tapered bit. Instead of clipping the two parts of that buckle together you clip one of the straps in there and wrap that around the seat post and cinch it up. Look at the second image from the “More images” bit here: https://www.alpkit.com/products/airlok-xtra-tapered
It’s all the same thing if you ask me. And it’s really just a case of deciding what you want to take on a particular trip and then deciding how you are going to carry it all. (top tip, what ever you do, it’s not on your back, I think we’ve all been there! 😆 )
Here’s a better pic, I add a shoelace 😀 to take the tension off the closure buckles. That strap, round the seatpost, and the other around the bag and saddle rails. Velcro loop bag’s nose to seatpost too.
My partner and I use the lightweight setup. We’ve gone on overnight trips and longer journeys of up to 3 weeks. I’ve got lots of tips, trips and gear reviews over on my blog including pack lists so you can see how much can be carried. Faraway Visions