These cages are based on an old French design from the '40s or early '50s. The Retro is a regular one-handed cage; you put in the bottle just like on 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, 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.