<label><input type="checkbox" data-bind="checked: displayMessage" /> Display message</label>
<div data-bind="if: displayMessage">Here is a message. Astonishing.</div>

ko.applyBindings({
displayMessage: ko.observable(false)
});

<ul data-bind="foreach: planets">
<li>
Planet: <b data-bind="text: name"> </b>
<div data-bind="if: capital">
Capital: <b data-bind="text: capital.cityName"> </b>
</div>
</li>
</ul>
<script>
ko.applyBindings({
planets: [
{ name: 'Mercury', capital: null }, 
{ name: 'Earth', capital: { cityName: 'Barnsley' } } 
]
});
</script>

<ul>
<li>This item always appears</li>
<!-- ko if: someExpressionGoesHere -->
<li>I want to make this item present/absent dynamically</li>
<!-- /ko -->
</ul>

<div data-bind="ifnot: someProperty">...</div>

<div data-bind="if: !someProperty()">...</div>