The bottom mounts with hook like the top. While the two top fasteners carry the load, the bottom merely prevents bags from swaying off rack sides.

Make sure your panniers are compatible with whatever racks that you choose, and that the racks are compatible with your bike.

Cheap bags use simple metal hooks with elastic for attachment. Fancy bags use hooks which lock to the rack tubing so that the bags can't be shook loose from rack.

The stronger, cheaper Tubus Cargo might be a better choice than the Airy. Ti tubing saves 250g but the rack is smaller, less rigid, harder to fit stuff to the very small top platform, and the 30kg rating is probably exaggerated a little. It's short in height too, don't plan on using very large tires/fenders with the Airy.

http://www.tubus.com/documents/1323435356_Airy_BM.pdf

http://www.tubus.com/documents/13365...Classic_BM.pdf