These cages are based on an old French design from the '40s or early '50s. In our estimation, they are the best looking bottle cage of all time.
The Retro is a regular one-handed cage; you insert the bottle just like in any other cage, but it holds the bottle very securely due to the spring-like shape. You can spread the cage for an oversized bottle, or squeeze the cage inward for a better fit with (for instance) a metal water bottle.
Retro cages are made by hand from a single length of 3mm stainless steel rod and then electro-polished. If you examine it closely from all angles you'll see that it is, indeed, hand made.