system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it that if my headphones are pulled slightly out of the port, the vocals will be cut out but the instruments can still be heard clearly.

When I use my iPod sometimes the headphone jack will be pulled very slightly out of the headphone port. This causes the vocals or one instrument to go silent while the other instruments remain at fill volume.
gpt
It's to do with something called 'phase'.  Imagine that the following characters are an image of a sound wave: 

nunununu

The noise that wave makes sounds identical to this wave:

unununun

It's the same wave,  we just started it in a different place. 

The funny thing is that if you add these waves together,  they cancel each other out.  The peaks and the troughs of the wave add up to zero. This is because they are precisely 'out of phase' with each other. 

When you pull out your plug a bit,  it reverses the polarity of the electrical signal to one ear,  which inverts the phase and so the waves cancel each other out. 

But why doesn't it just go silent? Good question. Because you're listening to a stereo signal,  the waves for each ear are different.  They can only cancel each other out if they are an exact copy,  exactly out of phase.  In most music,  the only thing that is not in stereo is the vocal,  which tends to be in the center of the 'sound image'.  That means that it's waveform is the same in both ears.  When you invert the waveform by pulling out the plug, it's the only thing that gets cancelled out! 

Actually,  you'll sometimes notice the bass line disappear, or other instruments that are centered in the sound image.