Jacquire King adds a third microphone behind an upright piano to capture the body and resonance that front-positioned mics tend to miss. Because the mic is seeing the instrument from the opposite side, King engages the polarity switch to keep it phase-coherent with the front pair, then uses an 1176 compressor to bring up the quieter signal and add a touch of control.
The rear mic produces a more muted, less transient-heavy sound on its own, but that's exactly what makes it useful for blending. King treats it as a separate texture layer rather than a replacement, dialing in only as much as the track needs.
From there, he routes the rear mic signal through a raw pedal and a delay to push it toward grit and character. The result is a blended piano sound that can range from open and natural to rough and atmospheric, all without committing to a single processing chain on the main signal.