Granted, I’m new to ham, but I haven’t seen a DIY j-pole like this with an element in the middle tuned for a different band. All the diagrams I’ve seen show that the 1/4 wave element (the parasitic element?) is supposed to cancel out the bottom part of the 3/4 wave (driven?) element.
At first I thought the 70cm band element was just for running a second coax connection that you’d swap between, but it has no connections. Furthermore, my reading on j poles says that they’re supposed to be sensitive to metallic stuff around them, so an extra element would seem to interfere.
A j-pole is a half-wavelength vertical with a quarter-wavelength matching section on the bottom.
It turns out that the 70cm band is about 3x the frequency of the 2m band (150MHz * 3 = 450MHz, close enough to each band). So the 2m the long leg of the j-pole is 3/4 wavelength (1/2 + 1/4 matching section), and on 70cm the long leg is 2.25 wavelength (3/2 + 3/4 matching). Both are an odd number of quarter waves, as we expect. The ham who made that briefing probably discovered in their testing that the matching stub wasn’t good for both bands, so they added a second one for 70cm.
This is not a novel design, Arrow Antennas has been selling one like it for years (https://www.arrowantennas.com/osj/j-pole.html)
