Hi sera, basically you just assign that class name to an element and then when you click on that element it will extend the page. For example the checkout page in the CMS uses it, so when you click the radio box to specify a different delivery address it displays extra fields on the page for this and because the radio box has class="extend_page" it extends the page, of course the tweak must be present in the page template. You can check it over on the Xara+CMS demo at
https://demo.xt-cms.com/xara/store/ - just add a product to cart and click through to the checkout page.
This is the HTML below for the radio box on the checkout page and you can see how it has class="extend_page"
<input type="radio" name="seperate_delivery_address" value="1" class="extend_page" onclick="javascript:show('delivery_details');">
So when this radio button is clicked it will show the extra fields for the delivery address and also extend the page. So using just this class name on an element with the tweak should extend the page with a click. Of course the javascript to show the extra/hidden content will vary depending on what you are using but the tweak and class name should work independently to that.
If you wanted to add this class name to something in Xara you can apply the name
htmlclass=extend_page using the apply name toolbar button or add it to custom HTML in a placeholder.