OnWorkBegin is a TWorkBeginEvent property that represents the event handler signalled when beginning a read or write operation on the connection. OnWorkBegin is generally used to initailize progress indicators or GUI components when an IO operation is started. 

OnWorkBegin is assigned to the TWorkBeginEvent event handler in IOHandler when the value for the IOHandler property is assigned. 

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

TWorkBeginEvent 

TIdIOHandler.OnWorkBegin 

IOHandler