What does this technique called? Also, what's the proper notation for this? The audio link is attached below.
Edit: It sounds like fast arpeggio repeated over and over again with the same voicing.
Adding to Aaron’s answer some notation remarks: In orchestral music such a between-two-notes tremolo is usually notated by notating the two notes in question with double length and putting tremolo beams between them.
In piano notation there is a special notation for this, where the tremolo beams attach to the stems of the notes, making it look like it was a pair of small notes with note heads from large notes. This is only really used for a total length written with a half note, as a whole note would look weird, and black notes allow for no distinguishing between smaller note values (so a 32th tremolo lasting a quarter would look just like 32th notes). Thus for such cases there is a notation where the first beam is attached to the stems, while the other beams are not.
Here is how these three variants look:
It's called a "shake", "tremolo", or "tremolando".
To get the specific effect, it's probably most clear to write "shake", "trem.", or "tremolo" above the notated chord.
"Tremolo bars" are also fine.
One can also use a trill line to indicate duration.