OnWork is a TWorkEvent property that represents the event handler signalled when read or write operations are invoked for the connection. OnWork is generally used to control the update of progress indicators or GUI components when an IO operation is performed. 

OnWork is assigned to the TWorkEvent event handler in IOHandler when the value for the IOHandler property is assigned. 

TIdTCPConnection contains an internal method normally assigned to OnWork. Applications can assign a procedure to the event handler to allow responsing the TWorkEvent notification. 

TWorkEvent 

TIdIOHandler.OnWork 

IOHandler