Here I describe various types of under the leg catches.
In one version you reach your arm under you leg to catch the disc.
In another version you swing your leg over the disc so that the disc travels under your leg while it is in flight. Then you catch it. James Wiseman would say this is technically a “the” catch since your hand is not restricted and it is not blind. However, I believe that the body mechanics and timing involved make this catch more difficult that the previous one.