“A mesostic is a poem or other typography such that a vertical phrase intersects lines of horizontal text. It is similar to an acrostic, but with the vertical phrase intersecting the middle of the line, as opposed to beginning each new line.
The practice of using index words to select pieces from a preexisting text was developed by Jackson Mac Low as “diastics”. It was used extensively by the experimental composer John Cage (Walsh 2001).”
This explanation is taken from the Wikipedia page on mesostics. The creator of ketsudo, Nicholas Wilson knew John Cage, and he introduced him to mesostics.
For those interested there are some extended mesostics on the blog and here’s a bit of philosophy.