location ~* \.(json)$ {
          echo "hello";
          echo_location_async /woot/blah;
          echo "goodbye";
        }

        location ~* /woot(.*)$ {
          echo $1;  # echos "/blah"
        }

----------------------------------------

        location ~* \.(json)$ {
          echo "hello";
          echo_location_async /woot$request_uri;
          echo "goodbye";
        }

        location ~* /woot(.*)$ {
          echo $1;  # starts downloading woot
        }