According to "http://www-128.ibm.com/developerworks/java/library/j-jsp04293.html"
"As its name suggests, flush indicates whether any existing buffer should be flushed before reading in the included content. The flush attribute is required in JSP 1.1, so you'll get an error if you leave it out of your code. In JSP 1.2, however, the flush attribute defaults to false. Because flushing is rarely a big concern, my advice is to leave flush set to true for JSP 1.1 and leave it off for JSP 1.2 and above."