In a standard humbucker the coils are connected in series, which means that from the live the signal goes through one coil and then through the other (in the opposite direction). When the wiring is parallel, the signal goes through both coils at the same time. This gives a much thinner scooped sound as the resistance is dropped massively (think about resistors to understand this, if you have 2 100k ohm resistors in series the resistance is 200k ohm, but in parallel it is only 50K ohm).
On BK's to wire in series the signal goes from red to green for coil 1, the green the connects to the white and flows to the black. Any other wiring will not give the full output of the pickup with hum cancelling. In parallel the red and white are live and green and black are ground. Split will either take the red/green or white/black as the only connections. Anything else is wrong, and will give you strange results.
