This section here:

https://github.com/xataio/pgroll/blob/9d2515963865ba4b5b71fe0ff9ea85e8ce9e2ddf/docs/README.md#drop-column

A drop column operation drops a column from an existing table.

drop column operations have this structure:

{
   "drop_column": {
    "table": "name of table",
    "column": "name of column to drop",
    "down": "SQL expression"
  }
}

It's not obvious to me what the "down" key here is used for.