It's phase cancelling. You can get the same thing, if you wore your speakers up wrong. Skylarking, by XTC, was originally out of phase. If you listen to the version with the shit drawing on a greeny blue background, the sound sort of disappears from the middle. The one with the woman's fanny moustache and the daisies is in phase and it sound a lot thicker.
On a guitar, if you've got a Gibson, with volumes for each pickup and you're on the middle and they're out of phase, rolling back the volume on either pot will start to thicken it up. I can't do that on a strat.
Which means that when the two pickups are wired out of phase together, any frequency that both of them make is silenced and all that's left are frequencies that either pickup makes, that the other one doesn't.
It sounds sort of chewy. It's a little bit quieter than in phase, but he dealt with that using the master volume at the front of his Gretsch.
I gather Jimmy Page used it quite a lot, too. I'm not a fan of Led Zeppelin, so I don't really know what he might have used it on; maybe it was mainly a live thing - like Squire. I think he used it on Elephant Stone, The Hardest Thing In The World and "Groove (Black Magic Devil Woman)" in studio recordings - and that's about it, maybe a little bit on This Is The One and little bits here and there. Both of those recordings were pre-Leckie, who didn't want it because it was "wooly". I'd be surprised if Leckie didn't know it was out of phase.
Once you know what it is, you'll hear it everywhere. It's all over Axis: Bold As Love and Electric Ladyland.
This message has been edited by pumpo on Jun 16, 2017 11:08 PM