When a part is recorded to a single stereo track only, it can sound pretty unnatural (sometimes even annoying - makes me feel like I'm half deaf...) on headphones. The only thing I'd really want to do to avoid this is mix the mono signal with the stereo, so that 100% right becomes e.g. 75% right and 25% left. (Fully mono would be 50/50 on all sounds, obviously).
Turns out, we have two left-right localization cues—intensity difference, and timing difference. Sounds on the left hit the right ear a bit late and filtered. This filter aims to reproduce that, effectively mimicking well-placed stereo speakers on headphones.
Then, it's time-reversed and the sign flipped, and an impulse is added at sample 0. This filter translates to mid/side stereo, applies this kernel to the side channel, then converts back to L/R for playback.
When a part is recorded to a single stereo track only, it can sound pretty unnatural (sometimes even annoying - makes me feel like I'm half deaf...) on headphones. The only thing I'd really want to do to avoid this is mix the mono signal with the stereo, so that 100% right becomes e.g. 75% right and 25% left. (Fully mono would be 50/50 on all sounds, obviously).