Update panel does the full postback

i have questions on Update panel.

1) people often use Update panel as a result when postback occur then flicker does not happen. so i am curious to know Update panel is fully ajax or just a eye wash ?

2) Update panel use xmlhttp for calling server side function ?

3) when we click button inside Update panel then what happen internally....partial postback like jquery ajax or full postback occur but cleverly gives partial postback feeling to use ?

please share the info about Update panel like how it works or is it really ajax or fake.