When engineering the absorber, Solar Hydronic Corp. created a tube-on-fin design for it's combination of performance and durability. Loose tube absorber designs produce the least effective heating surfaces, and flat plate and welded tube designs don't allow for expansion and contraction leading to premature failure.
Simply put, tube-on-fin absorber design takes the best of previous designs, and combines them, while omitting all their flaws.
The fin, or webbing between each absorber tube, is set low, which allows the tube to capture more energy from the sun. The fin is also curved downward between each tube, raising the tubes slightly off the roof, while allowing the roof to "breathe". The web also affords a stress free flexibility to the tubes. This ensures there is no abrasion between the water carrying vessel and your roof.