With the guitar feedback trick, just like a microphone, many guitars will start up a howl, often involving the strings, when pointed at the speaker from only a couple feet away. A phenomenon, when it's kept under control, sometimes used to good effect by the guitarist, but when it's unexpected - not. Whereas, with the player's body between the guitar and speaker, and pickups pointed away from the amp, less chance of setting up that feedback loop. Polarity of the speaker or pickup again has little to nothing to do with the fb loop. Instead proximity, directionality and frequency response are the guiding factors.