The upper part of the jig appears to serve to press the two halves of the blocks together, and to align the guide horns. I found the horns would stick inside the upper half and you can just press down the block halves with your fingers anyway. Plus, if you assemble the tracks as instructed, you will end up with a whole bunch of sections of six that you then have to join. With my way, you just keep extending the track by seven links at a time, which is less fiddly.