In the words of Seymour Duncan...

In the past, if you wanted humbucker tone, P90 tone, and Strat tone, you would have needed three different guitars. But now, the P-Rails humbucker delivers all three sounds in a pickup that is a drop-in replacement for any standard-sized humbucker slot. Using any 3-way (on-off-on) mini-switch, each unique hybrid humbucker can be split to give you a super fat, authentic vintage P90, or be split to the Rail coil for a classic Strat-like tone.

 With both coils on, you get a big-sounding humbucker that packs a nice full punch. The neck version is wound RWRP so that, you can use both P90s or both Rails together and achieve hum-canceling operation.