Weirdly enough, the DiskFiltration hack, as it's called, works by taking control of a hard drive's actuator. Much like the arm of a record player it moves back and forth across a hard drive's platters to read and write data, and while it moves it makes a series of various sounds. Using the right type of malware, hackers can use those very sounds for some extremely shady machinations. If the right person is listening for the right thing, those noises can let go of a lot of information like an encryption key.
The DiskFiltration method only works up to about six feet and it's limited to a data rate of about 180 bits per minute. That's obviously not very fast at all, but it's quick enough to capture an encryption key in about a half hour. The entire idea and method of execution is massively impractical but it does work, except with solid-state drives. Still, it's a very weird and very real way of intercepting data. As if people needed another way to do so.