The minimising rules applied are listed here:
http://www.crockford.com/javascript/jsmin.html
JSMin is a filter that omits or modifies some characters. This does
not change the behavior of the program that it is minifying. The
result may be harder to debug. It will definitely be harder to read.
JSMin first replaces carriage returns ('\r') with linefeeds ('\n'). It
replaces all other control characters (including tab) with spaces. It
replaces comments in the // form with linefeeds. It replaces comments
in the /* */ form with spaces. All runs of spaces are replaced with a
single space. All runs of linefeeds are replaced with a single
linefeed.
It omits spaces except when a space is preceded and followed by a
non-ASCII character or by an ASCII letter or digit, or by one of these
characters:
\ $ _
It is more conservative in omitting linefeeds, because linefeeds are
sometimes treated as semicolons. A linefeed is not omitted if it
precedes a non-ASCII character or an ASCII letter or digit or one of
these characters:
\ $ _ { [ ( + -
and if it follows a non-ASCII character or an ASCII letter or digit or
one of these characters:
\ $ _ } ] ) + - " '
No other characters are omitted or modified.
There are other custom formatting rules applied according to the plugin developer's page:
http://www.sunjw.us/jsminnpp/