how the triger works in asp.net

<%@ Page Language="C#" MasterPageFile="~/MiniErpMaster.master" AutoEventWireup="True"
    Inherits="ItemMaster" CodeBehind="ItemMaster.aspx.cs" %>

<asp:Content ID="Content2" ContentPlaceHolderID="ErpMainContent" runat="server">
    <asp:UpdatePanel ID="Main" runat="server" UpdateMode="Conditional" ChildrenAsTriggers="true">
        <Triggers>
            <asp:AsyncPostBackTrigger ControlID="BtnSave" EventName="OnClick" />
        </Triggers>
        <ContentTemplate>
            <div class="wrap">
                <table width="100%">
                    <tr>
                        <td align="center" class="GenericBackgroud">
                            Item Master
                        </td>
                    </tr>
                </table>
                <table width="100%">
                    <tr>
                        <td>
                            <table>
                                <tr>
                                    <td class="tdlablesize">
                                        <asp:Label ID="lblItemName" runat="server" Text="Item Name"></asp:Label>
                                    </td>
                                    <td class="tdcustom">
                                        <asp:TextBox ID="txtItemName" runat="server" CssClass="textbox" TabIndex="1"></asp:TextBox>
                                        <asp:RequiredFieldValidator ID="rfvItemName" ForeColor="Red" ControlToValidate="txtItemName"
                                            runat="server" Text="Required field"></asp:RequiredFieldValidator>
                                    </td>
                                    <td class="tdlablesize">
                                        <asp:Label ID="lblItemType" runat="server" Text="Item Type"></asp:Label>
                                    </td>
                                    <td class="tdcustom">
                                        <asp:TextBox ID="txtItemType" runat="server" CssClass="textbox" TabIndex="2"></asp:TextBox>
                                    </td>
                                    <td class="lablecolor">
                                        <asp:CheckBox ID="chkIsResaleItem" Text="Resale Item" runat="server" TabIndex="5" />
                                    </td>
                                </tr>
                                <tr>
                                    <td class="tdlablesize">
                                        <asp:Label ID="lblItemVolume" runat="server" Text="Item Volume"></asp:Label>
                                    </td>
                                    <td class="tdcustom">
                                        <asp:TextBox ID="txtItemVolume" runat="server" CssClass="textbox" TabIndex="3"></asp:TextBox>
                                        <strong>ml<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server"
                                            ErrorMessage="Numbers Only" ControlToValidate="txtItemVolume" ForeColor="Red"
                                            ValidationExpression="^\d+$">
                                        </asp:RegularExpressionValidator>
                                        </strong>
                                    </td>
                                    <td class="tdlablesize">
                                        <asp:Label ID="lblItemCode" runat="server" Text="Item Code"></asp:Label>
                                    </td>
                                    <td class="tdcustom">
                                        <asp:TextBox ID="txtItemCode" CssClass="textbox" runat="server" TabIndex="4"></asp:TextBox>
                                        <strong>
                                            <asp:RegularExpressionValidator ID="RegularExpressionValidator2" runat="server" ErrorMessage="Numbers Only"
                                                ControlToValidate="txtItemCode" ForeColor="Red" ValidationExpression="^\d+$">
                                            </asp:RegularExpressionValidator>
                                        </strong>
                                    </td>
                                    <td>
                                        <asp:TextBox ID="txtItemNbr" runat="server" CssClass="textbox" Visible="False"></asp:TextBox>
                                    </td>
                                </tr>
                                <tr>
                                    <td>
                                    </td>
                                </tr>
                            </table>
                        </td>
                    </tr>
                </table>
            </div>
            <br />
            <table width="100%">
                <tr>
                    <td align="center">
                        <asp:Button ID="BtnSave" runat="server" Text="Save" OnClick="BtnSave_Click1" TabIndex="6"
                            CssClass="button" />
                        <asp:Button ID="btnUpdate" runat="server" Text="Update" CssClass="button" OnClick="btnUpdate_Click"
                            TabIndex="7" />
                    </td>
                </tr>
            </table>
            <br />
            <br />
            <br />
            Item Master Details
            <br />
            <div class="divCSSforGirdView">
                <fieldset>
                    <asp:GridView Width="100%" ID="grdvwItemMasterList" runat="server" AutoGenerateColumns="false"
                        OnRowCommand="CustomersGridView_RowCommand" OnRowEditing="grdvwItemMasterList_RowEditing"
                        OnRowDeleting="grdvwItemMasterList_RowDeleting" OnSelectedIndexChanged="grdvwItemMasterList_SelectedIndexChanged"
                        TabIndex="8">
                        <SelectedRowStyle BackColor="#99CCFF" />
                        <Columns>
                            <asp:TemplateField>
                                <ItemStyle Width="80px" />
                                <ItemTemplate>
                                    <asp:LinkButton ID="lnkDelete" runat="server" CommandName="Delete" Text="Delete"
                                        CommandArgument='<%# Eval("ItemNbr") %>' OnClientClick="javascript:return confirm('Do you really want to \ndelete the item?');"
                                        CausesValidation="false">
                                    </asp:LinkButton>
                                </ItemTemplate>
                            </asp:TemplateField>
                            <asp:ButtonField ButtonType="Link" Text="Edit" CommandName="Edit" />
                            <asp:BoundField DataField="ItemNbr" HeaderText="Item Number" ReadOnly="true" />
                            <asp:BoundField DataField="ItemName" HeaderText="Item Name" ReadOnly="true" />
                            <asp:BoundField DataField="ItemType" HeaderText="Item Type" ReadOnly="true" />
                            <asp:BoundField DataField="ItemVolume" HeaderText="Item Volume" ReadOnly="true" />
                            <asp:BoundField DataField="ItemCode" HeaderText="Item Code" ReadOnly="true" />
                            <asp:BoundField DataField="IsResaleItem" HeaderText="Is Resale Item" ReadOnly="true" />
                        </Columns>
                        <FooterStyle CssClass="FooterStyle" />
                        <RowStyle CssClass="RowStyle" />
                        <EmptyDataRowStyle CssClass="EmptyRowStyle" />
                        <PagerStyle CssClass="PagerStyle" />
                        <SelectedRowStyle CssClass="SelectedRowStyle" />
                        <HeaderStyle CssClass="HeaderStyle" />
                        <EditRowStyle CssClass="EditRowStyle" />
                        <AlternatingRowStyle CssClass="AltRowStyle" />
                    </asp:GridView>
                </fieldset>
            </div>
        </ContentTemplate>
    </asp:UpdatePanel>
</asp:Content>