At the other end of the spectrum of polar patterns the figure-of-eight microphone is sensitive to the pressure gradient of the sound wave.
The diaphragm is completely open to the air at both sides.
Even though it is very light and thin, there is a difference in pressure at the front and rear of the diaphragm, and the microphone is sensitive to this difference.
The pressure gradient is greatest for sound arriving directly from the front or rear, and lessens as the sound source moves round to the side.
When the sound source is exactly at the side of the diaphragm it produces equal pressure at front and back, therefore there is no pressure gradient and the microphone produces no output.
Therefore the figure-of-eight microphone is not sensitive at the sides. (You could also imagine that a sound wave would find it hard to push the diaphragm sideways sometimes the intuitive explanation is as meaningful as the scientific one).
All directional microphones exhibit a phenomenon known as the proximity effect or bass tip-up.
The explanation for this is sufficiently complicated to fall outside of the required knowledge of the working sound engineer.
The practical consequences are that close miking results in enhanced low frequency.
This produces a signal that is not accurate, but it is often thought of as being warmer than the more objectively accurate sound of an omnidirectional microphone.