In Rautavaara's Second Piano Sonata, first movement, beginning 20 bars before the end, there is a tablature-like symbol. What does it mean?
The symbol denotes a tone cluster that includes all chromatic pitches between D1 and D2. The notation is explained in Rautavaara's "Table of Clusters".1
The effect can be heard in this recording by Laura Mikkola. The link is timed to the cluster. The "Table of Clusters" is shown at the beginning of the video.
Another example of this notation can be found at the end of (the piano arrangement) of Danny Elfman's "Victor's Piano Solo" from The Corpse Bride.2
(Arguably, the notation above is not an exact representation of the piece as heard in the movie. That specific effect could be notated with the cluster chord indication plus a downward-pointing arpeggio sign.)
1 Zachary Matthew Ridgeway, "The Fire Sermon: Program and Narrative in Einojuhani Rautavaara’s Second Piano Sonata", PhD Dissertation (University of Texas, 2018). (Accessed 29 Jan 2021.)
2Danny Elfman, "Victor's Piano Solo (from The Corpse Bride)", in The Halloween SongBOOk (Alfred Publishing, n.d.); song is (c) 2005 by Warner-Barham Music LLC.