Check the article for the full context. It’s basically a musical version of exquisite corpse. One composer wrote the start of a phrase (on sheet music, without using a piano), then passed it on to the next composer, who continued the phrase based only on reading the sheet music.

This illustrates the extraordinary degree of audiation, or the cognitive ability to imagine music in the mind in a symbol-linked, systematized way, that used to exist in classical music.

Each composer could read what was just written, audiate how it would sound, audiate a continuation of the phrase, and then transcribe that aural imagination back to sheet music.

Classical jamming.