From the jsp 2.0 spec
The flush attribute controls flushing. If true, then, if the page output is buffered and the flush attribute is given a true value, then the buffer is flushed prior to the inclusion, otherwise the buffer is not flushed. The default value for the flush attribute is false.
forward doesn't let you write something before forwarding. So it's of no use in a forward action
More on the use of flush in include action please read this
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