AJAX :: UpdatePanel: What Is The Purpose Of PostBackTrigger

In documentation I have read:

„Use the PostBackTrigger control to enable controls inside an UpdatePanel to cause a postback instead of performing an asynchronous postback."

I made simple application and I do not see difference between using PostBackTrigger and standard postback.

I have three buttons: two inside update panel and one outside update panel. When I click btnUpdateAsync in event handler I set text property of lblUpdatePanel and lblNoUpdatePanel and only lbl that is inside update panel is chaned if web browswer - so this is ok.

Whant I click btnUpdateInUpdatePanelSync all page is refreshed and both lablels are changed in web browser.

When I click btnUpdateOutsideUpdatePanel I see the same behavior.

If I used button that is not associated with PostBackTrigger I would get the same behavior - so what is the purpose of PostBackTrigger?

This is my app:

[Code]....

protected void Page_Load(object sender, EventArgs e)
{
}
protected void LinkButton1_Click(object sender, EventArgs e)
{
}
protected void btnUpdateAsync_Click(object sender, EventArgs e)
{
lblUpdatePanel.Text = "btnUpdateAsync_Click";
lblNoUpdatePanel.Text = "btnUpdateAsync_Click";
}
protected void btnUpdateInUpdatePanelSync_Click(object sender, EventArgs e)
{
lblUpdatePanel.Text = "btnUpdateInUpdatePanelSync_Click";
lblNoUpdatePanel.Text = "btnUpdateInUpdatePanelSync_Click";
}
protected void btnUpdateOutsideUpdatePanel_Click(object sender, EventArgs e)
{
lblUpdatePanel.Text = "btnUpdateInUpdatePanelSync_Click";
lblNoUpdatePanel.Text = "btnUpdateInUpdatePanelSync_Click";
}