Yeah, it is amazing how many people are surprised by things that are in the instructions. They're not hidden features.
In this case the instructions were not totally clear, but the side-on photos on the instructions were. I felt like when I pushed it all the way up that it was getting jammed on something as I was squeezing the backpack together, which is why I was looking online. Eventually I found someone to say that what I was doing was correct, it just needed a bit more oomph.