I think the effect you're talking about is right after the main verse riff kicks in ... full distortion, etc ...... Jimi bends the g-string at the 2nd fret up a whole-step and while slowly lowering it back to pitch, rocks his pickup selector switch back and forth between the bridge and neck pickups several times. Do this with a ton of distortion and maybe a flanger or phaser pedal and you will get the picture ..