OnReceive is a TIdInterceptStreamEvent property that represents the event handler signalled when a read operation needs to be performed for the Connection in the intercept. 

OnReceive is triggered in the Receive method, and allows handling or transformation of data in the Receive buffer. 

Applications must assign a procedure to the TIdInterceptStreamEvent event handler to allow responding to the event notification. 

Data can be modified in OnReceive, but it will have already been passed to an assigned Intercept. 

TIdInterceptStreamEvent 

TIdConnectionIntercept.Receive