Edit in JSFiddle

<label>
    <input type="checkbox" data-bind="checked: hasChecked" />Tick me!
</label>
<div data-bind="visible: hasChecked">
    Hey! Watch out!
</div>

function ViewModel() {
    var self = this;
    self.hasChecked = ko.observable(false);
}

ko.applyBindings(new ViewModel());