understanding post-cache substitution control

 
Public Shared Function DoMe(ByVal context
As HttpContext) As String
    Dim b As New Button()
    b.ID = "btn1"
    b.Text = "Hello"
   
    Dim sb As New System.Text.StringBuilder()
    Dim sw As New System.IO.StringWriter(sb)
    Dim hw As
System.Web.UI.HtmlTextWriter = New HtmlTextW ...