By removing the adjustable poles on a humbucker can reduce the output and fullness of the pickup. Removing the adjustable poles keeps the magnetic field from passing through one of the bobbins and to the strings. The magnet magnetizes the adjustable pole piece and directing the magnetic field to the string. The pickup will still work because of the ferrous slug pole piece in the other bobbin that isn’t normally not seen when the cover is in place but can be seen when the cover is removed. The ferrous slugs conduct the magnetic field to the strings and will still give you output similar to a single coil pickup.