I have been wondering the same thing, and posted a similar question about 8 months ago. As far as I can tell, nobody actually knows how to use those properties, and I have been unable to find any mention of them in any documentation. I was able to "brute-force" my database updates using TMySQL50Connection.ExecuteDirect, although I am reasonably sure that is not what the designer intended. I got that far by poking around in the source code for unit db.
As for what the designer of the TSQLdb components actually intended for the DeleteSQL, UpdateSQL, and InsertSQL, that appears to be a deep, dark secret.