<%@ Register Assembly="AsyncControls" Namespace="DelvingWare.AsyncControls" TagPrefix="dw" %>

<dw:AsyncCheckBox runat="server" ID="chkMain" 
    Text="Check the box" OnCheckedChanged="chkMain_Checked" />

<dw:AsyncLabel runat="server" ID="lblMain" 
    Text="Check or uncheck the AsyncCheckBox" RenderMode="H4" />

using System;
using DelvingWare.AsyncControls;
...
protected void chkMain_Checked( object sender, AsyncEventArgs ae )
{
    // update the label
    if ( chkMain.Checked )
        lblMain.Text = "AsyncCheckBox is checked.";
    else
        lblMain.Text = "AsyncCheckBox is not checked.";
}

Imports System
Imports DelvingWare.AsyncControls
...

Protected Sub chkMain_Checked(ByVal sender As Object, ByVal ae As AsyncEventArgs)
    ' update the label
    If chkMain.Checked Then
        lblMain.Text = "AsyncCheckBox is checked."
    Else
        lblMain.Text = "AsyncCheckBox is not checked."
    End If
End Sub

.asyncChkBox1 {
    font-size: 8pt;
    font-family: Verdana;
}