test.hbs

<!--- the consuming component --->
{{#needy-component as |sub-component|}}
<h1>Hello</h1>

<div>
  {{sub-component}}
</div>
<!--- ... -->
{{/needy-component}}




<!-- needy component's template --->
{{yield}}
{{! # magic sub-component }}
<p>World</p>
{{! /magic }}




<!-- resulting HTML --->
<h1>Hello</h1>

<div>
  <p>World</p>
</div>