Output sub nav from related nav item, from the top level entry

gistfile1.twig

{# Get the related nav item for the current entry #}
{% set navItem = craft.entries.section('navigation').relatedTo(entry).first() %}
{% if navItem %}
	{# Get the top level page for the nav item #}
	{% set topPage = navItem.getAncestors().level(1).first()|default(navItem) %}
	{# Build a "pages" array w/ the top page and its descendents (child entries) #}
	{% set pages = craft.entries.id([topPage.id]|merge(topPage.getDescendants().ids()|default([]))) %}
	<nav>
	    <ul>
	        {% nav page in pages %}
	        	<li>
	                <a href="{{ page.singleEntrySelector.first().url | default }}">{{ page.title }}</a>
	                {% ifchildren %}
	                    <ul>
	                        {% children %}
	                    </ul>
	                {% endifchildren %}
	            </li>
	        {% endnav %}
	    </ul>
	</nav>
{% endif %}