Edit in JSFiddle

var viewModel = {
    detailsEnabled: ko.observable(false),
    enableDetails: function () {
        this.detailsEnabled(true);
    },
    disableDetails: function () {
        this.detailsEnabled(false);
    }
};
ko.applyBindings(viewModel);

<div>
    <div data-bind="event: { mouseover: enableDetails, mouseout: disableDetails }">Mouse over me</div>
    <div data-bind="visible: detailsEnabled">Details</div>
</div>