If you've been researching the stacked non-fouler mod as well as my angle block, you probably have a good idea of how this works. When you add a high flow cat and larger diameter exhaust, the secondary O2 sensor "sees" more exhaust and
then tattles back to the computer since it thinks something is wrong. By merely pulling the nose of the sensor (secondary--never primary) out of the direct exhaust stream, it basically simulates having the smaller exhaust/OEM cat again. The beauty is that it is a mechanical fix--if it needs to be set back to stock, it can be
with the rockauto bits, the top of the two spacers needs widening about 15mm to take the lamda probe
hth