A page is sent in one piece, once all of it is rendered. {flush} sends what is rendered so far straight away: put after the wrapper's </head>, it lets the browser fetch the stylesheets and fonts while a slow part of the page is still being rendered.

<p>The top of the page, sent at once.</p>
{flush}
<p>The rest, sent when it is rendered.</p>

Shown here inside another page, the fragment flushes nothing - only the page that is the response does. Asked for on its own, it sends its first line before the second is rendered; either way the page arrives whole.

• Everything that needs the whole page is off for a request that flushes: tidy, gzip of the whole body, the ETag and its 304, Content-Length, the page cache. The headers go out with the first part.
• It stands at the top level of the page or its wrapper - not inside another tag. The page's PHP runs before any of the template, so what a flush wins is the rendering below it: a big table, a {curl}, a select.