You're likely right Haifisch since spectra isn't electrically conductive. The black box manual I read stated that the reason it was effective is because the wire cable of the downrigger traveling through the water and ultimately through the earths magnetic fields, not matter how slow, produced a small amount of electrical current in much the same way as a generator coil passing through magenetic fields. Although almost imperceptable (except to lab standard test instrumentation) it's like a cattle prod to a fish, which is why the black box works. Now if all that is true, when you switch to spectra line you wouldn't need the black box anymore, and the lack of electrical conductivity in the spectra could be a real advantage if you own an aluminum boat. My .02 cents!