Well, the trailer does a few things for a jig, first, it adds bulk, making it appear larger, and slowing the fall, second, it's for looks, the claws sticking out the back resemble a crawfish. Third (and not lastly), it provides action, via the claws flapping about.
I made that jig and trailer, the skirt was in pad form, I combined them in the 'arm' part that acts as a rattle holster. What the pads are basically is a 5.5" piece of silicone that has slits, but is bonded on each end for ease of assembly. On that particaular type of skirt band, I slip it on the nose of a pair of needle nose pliers, then fold the bonded ends of the pad together to place in the band, then pull them through. Once I have the band positioned where I want it, you simply cut the bonded ends, freeing the slit rubber, most have around 20 strands. Skirts also come pre-made, with the band already on them, but if you went as far as making the jig, why limit yourself!