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->title and $document->content are variables containing preformatted HTML which must be echoed as is.