Transaction Group Table Action Custom Update

What is the syntax for the custom update/select section of a standard transaction group? Do you enter a WHERE clause? Can it be complicated? One example is I would like to update the most recent row in a table that matches a criterion. There may be several rows in the table that match the criterion, but I want to update the most recent (as measured by a tstamp or by the primary key).

Bump. Also trying to figure out how to use custom update query for block transaction groups.
Pretty poor documentation on it