PostBackTrigger in ASP.Net

<asp:Panel ID="panel_Buttons" runat="server">
                <div class="btnslist">
                <asp:UpdatePanel ID="UpdatePanel_Buttons" runat="server">
                <ContentTemplate>
                    <asp:Button ID="btnSave" runat="server" CssClass="btns" OnClick="btnSave_Click" Text="Save as Draft" Visible="false"/>
                </ContentTemplate>
                <Triggers>
                    <asp:PostBackTrigger ControlID="btnSave" />
                </Triggers>
                </asp:UpdatePanel>
                </div>
            </asp:Panel>