OnPress(bool pressed) is called by the NGUI event system, which is triggered by you touching the screen.
The code saves when it is pressed down.
It does stuff in Update() which is called every frame if the code is in PressedDown state (mIsPressed == true).
When you lift your finger, a OnPress(false) call is made by the event system, which sets mIsPressed to false, which stops the update loop from doing its thing.
Does that explain it to you?