Calf moving forward and into chair seat is incorrect, but that's not what I get from that diagram.
I get that you stretch down into your heel so your ankle can act as a shock absorber, not that you're pushing the leg forward.
Check out any of George Morris's books, Gordon Wright's Learning to Ride, Hunt and Show for good references on jumping position.
You can also go the Practical Horseman website, and check out George Morris's monthly column on jumping. Reading a half dozen of his monthly columns will give you a good working knowledge of functional jumping position.