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

OnSend is triggered in the Send method, and allows handling or transformation of data in the Send buffer. 

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

Values in the Send buffer are made available to an assigned Intercept after OnSend has completed.

TIdInterceptStreamEvent 

TIdConnectionIntercept.Send