There are a bunch of photos showing the test fixtures that they're using to measure frame flex which pretty clearly show what they're constraining and where they're adding force with the cylinder. It's not a free body diagram and it doesn't show the rider, but what exactly are you looking for?
Well, diagrams. Apply this force, get this displacement, this is how wheels are out of plane (disregarding that turning makes them out of plane already).
None of the pictures are labeled, and they only show one setup: pressing the pedal without sitting on the seat, without holding the handlebars, while both wheels remain in plane. I'm not sure if it's even possible to keep both wheels in plane when pedalling on a moving bicycle, if only due to pressing on the handlebars.
