It's quite simple really. The text input defines a separator for parts of the search term. It's like the first parameter to split. If you say your separator is an ; and enter " on ;foo" in the search box, it will search for the string " on " and the string "foo", where the latter may be embedded in other words like "stfoobar".

With the radio buttons, you just choose whether the individual parts of the search term (after splitting) are treated as plaintext words or regexen.

Makeshifts last the longest.