it was running reverse polarity on one coil in series with the other, the second positive became a negative, so the coils were acting against each other, one was pushing out when the other was pushing in, it would work. mot that difficult, electricity flows neg to positive but we will think in reverse since he did it in reverse. sp imagine the + wire all the way to the right is the pos of the amp, it then runs through that coil and out the negative, now normally to run series you would jump over to the positive of the coil but instead he jumped over to the negative of the coil and out the positive back to the negative of the amp groinding out the voltage. so now he has one coil in reverse polarity and one in normal.