The OnProgress event is automatically called from a Write operation, after the object's internal buffer has been filled and written to the output stream.  

The OnProgress event is of type TNotifyEvent. When your procedure is called, the Sender parameter will be set to the object itself. You can use the OnProgress event to update a user interface control, such as a progress indicator. You can use the Position property, inherited from TStream, to determine the number of raw (uncompressed) bytes written to the output stream so far.