<textarea class="form-control" placeholder="Comments" rows="10" data-bind="value: $root.GetTabComment($data).Comment, valueUpdate: 'keyup'"></textarea>

Comment.subscribe(function(){
  //save code
});

valueUpdate: 'input'

<p>Type stuff here:
    <input data-bind='value: instantaneousValue, valueUpdate: "input"' /></p>

<p>Current throttled value: <b data-bind='text: throttledValue'> </b></p>

<div data-bind="visible: loggedValues().length > 0">
    <h3>Stuff you have typed:</h3>
    <ul data-bind="foreach: loggedValues">
        <li data-bind="text: $data"></li>
    </ul>
</div>

function AppViewModel() {
    this.instantaneousValue = ko.observable();
    this.throttledValue = ko.computed(this.instantaneousValue)
                            .extend({ throttle: 400 });

    // Keep a log of the throttled values
    this.loggedValues = ko.observableArray([]);
    this.throttledValue.subscribe(function (val) {
        if (val !== '')
            this.loggedValues.push(val);
    }, this);
}

ko.applyBindings(new AppViewModel());