When you use both pickups in a Fender Jazzmaster the sound is humbucking and very quiet. The bridge pickup is wound counter-clockwise with South polarity and the neck pickup is wound clockwise with North polarity. When both pickups are used together, they become connected in parallel and become humbucking because each have an opposite winding direction and magnetic polarity. When each pickup is used individually it is still a single coil pickup and prone to hum from stray electrical noise and magnetic fields.