trying to understand postback AND placeholder.dropdownlist

************** I have the following placeholder: ***********************

<asp:PlaceHolder ID="placeholderexpertise" runat="server"></asp:PlaceHolder>

************* Then, within my aspx.cs file I put a dropdownlist in that placeholder, during Page_Load: ************************

/** create the dropdownlist for this specific time through the loop **/

if (!this.IsPostBack)

{

for (i = 0; i < intcountofexpert ...