In this mode, fax pages are sent in blocks and for any block that fails the error check, the receiver will ask the sender to send it again until all blocks have been received successfully. ECM automatically detects and corrects errors in the fax transmission process that are sometimes caused by telephone line noise.
However, when ECM is unavailable, there is no way for the receiver to request part of the page that has not been received correctly. Turning off ECM might improve the ability to send a fax successfully when the phone line quality is poor, but the image quality of the received fax might also be poor and/or missing parts of pages.
To find out how to enable ECM on your printer/fax machine consult your owners manual for your machine, or search online.