how do we use it as such?

Naturally, you surround the part(s) to be deleted with <readmore> and </readmore>, as you would see if you did "view HTML source" on that node.

How long has it been available in this enhanced form

BTW, I think it would be good to make the readmore tags somehow subtly visible (without having to resort to "view source") when viewing the full node, but I haven't come across an appealing presentation for that (none have been nearly subtle enough).

Update: No, wait. I did come up with one I liked, but I had forgotten it. My idea was to provide a "Read less" link in the node header if a <readmore> is present and that would add a readless=1 CGI parameter that would trigger readmore processing even though you are looking directly at the node. Anyone game?

I had assumed that the readmore tag would not show up in the delivered page, any more than a code tag or square bracket syntax. What we type in the "Your text:" submission is different from what is sent to the browser, so I assumed readmore worked the same way.

Also try the "xml" view (and "view source" on it).

        - tye (TMTOWTVI)