This bit breaks down the figure-8 (bi-directional) polar pattern, common on ribbon mics and selectable on many large-diaphragm condensers like the 414 shown here.
Picture two cardioid bubbles fused back to back: sound is picked up equally from the front and rear of the mic, with nothing captured from the sides.
The key insight is the null point. Directly above, below, and to the sides of the capsule, the figure-8 pattern rejects sound almost completely.
That null zone gives you a practical tool for reducing bleed. Instead of only aiming the mic at what you want to capture, you can angle those null points at an amp, a monitor, or another instrument you want to keep out of the signal.