If the introtext field is empty (default is an alias of isempty), it outputs the first 400 characters of the content field, followed by three dots. Without the content part, all you'd see is the three dots.
It's somewhat dangerous, because if there is a tag that's opened in the first 400 characters of the content field, but not closed, it can break the page. You can add the :striptags filter to prevent that. I think it goes like this:
[[+introtext:default=`[[+content:striptags:ellipsis=`400`]]`]]
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