This is not an expression modifier but a keyword.
While printing variables inside PHPTAL templates, you will have noticed that PHPTAL encodes each variable to ensure the validity of the output document.
Sometimes, you may use HTML/XML variables which must be echoed as is.
<h1 tal:content="structure document/title"/> <span tal:replace="structure document/content"/>
In above examples, we assume that
$document->content are variables containing preformated HTML which must be echoed as is.