From watching the video it looks like that pedal releases a bunch of tied sheaves from the cradle which carries them on the right hand side of the machine. This saves walking up the field collecting individual sheaves from the stubble so as you can make a shock.
A manual release allows the operator to release all the bunches from subsequent passes next to each other in a line across the field, making them easier to collect with a wagon, as the shocks are all in a line.
I think on that machine the pedal is sticking, you can see the operator bending forward and reaching to pull it up after releasing the sheaves, or maybe it works the other way around and releases the cradle when he lifts his foot.
That's what I think, unless anyone knows better. I haven't actually looked at a Mc Cormick binder closely to see and be dead sure of it. I haven't personally operated one, but have seen them working quite a few times.
