The middle Stratocaster pickup is typically reverse-wound and reverse-polarity (RWRP) so that when using a 5 way switch and you are in the 2 or 4 position it creates a hum canceling effect. This is achieved by having the middle pickup's winding direction and magnet polarity be the opposite of the neck and bridge pickups. The hum-canceling capability provides noise reduction, while the unique "out-of-phase" sound in these positions is the classic Stratocaster Quack
Hum-Canceling: When two single-coil pickups are on, they can pick up 60-cycle hum. The RWRP design cancels this out because the middle pickup's electrical and magnetic signals are opposite to those of the other two pickups, effectively canceling each other out.
Quack sound: This hum-canceling effect, when combined with the two pickups, produces the signature, slightly thinner "quack" or "in-between" tone that many guitarists love.
Reverse-wound: The wire is wound in the opposite direction compared to the neck and bridge pickups.
Reverse-polarity: The magnetic polarity of the pickup is flipped (the north poles on the top are now south).
Modern vs. vintage: While modern Stratocasters come with RWRP middle pickups from the factory, vintage models did not. Older models used three identical pickups, resulting in hum in the middle positions and requiring players to avoid the "in-between" sounds