<p>Send me spam: <input type="checkbox" data-bind="checked: wantsSpam" /></p>
<div data-bind="visible: wantsSpam">
    Preferred flavors of spam:
    <div><input type="checkbox" value="cherry" data-bind="checked: spamFlavors" /> Cherry</div>
    <div><input type="checkbox" value="almond" data-bind="checked: spamFlavors" /> Almond</div>
    <div><input type="checkbox" value="msg" data-bind="checked: spamFlavors" /> Monosodium Glutamate</div>
</div>
 
<script type="text/javascript">
    var viewModel = {
        wantsSpam: ko.observable(true),
        spamFlavors: ko.observableArray(["cherry","almond"]) // Initially checks the Cherry and Almond checkboxes
    };
 
    // ... then later ...
    viewModel.spamFlavors.push("msg"); // Now additionally checks the Monosodium Glutamate checkbox
</script>