Taken from the Sprockets 1.02 github (Sprockets 2 is what rails 3.1 uses to accomplish asset loading):
How Sprockets handles comments
Use single-line (//) comments in
JavaScript source files for comments
that don't need to appear in the
resulting concatenated output.Use
multiple-line (/* ... */) comments for
comments that should appear in the
resulting concatenated output, like
copyright notices or descriptive
headers. PDoc (/** ... **/)
documentation comments will not be
included in the resulting
concatenation.
Comments beginning with //= are
treated by Sprockets as directives.
Sprockets currently understands two
directives, require and provide.
What this means is that //= jquery_ujs
is a directive. It instructs Sprockets to include the jquery_uls file when it compiles all the jquery files.
If you don't want that file included, just remove the equals sign and the directive becomes a comment.