It works like this: Two people, or a group of people use the same e-mail account. The account is usually a Gmail, Yahoo or other free service. Rather than sending emails, messages are saved as drafts. When the other person checks email, they look at the 'Drafts' folder to find the message. Once the message has been read it is deleted from the account which leaves no trace.
Anyone gaining access to the account will see no evidence of draft messages as they do not stay on the account, even in a deleted messages folder.
The tactic has been used for over a decade when it was revealed that terrorist organizations used the trick to avoid their communications from being intercepted by the FBI.
Former CIA director David Petraeus famously used email drafts to communicate with a woman he was having an affair with, his biographer Paula Broadwell.
Members of Al-Qaeda were known to use email drafts as far back as 2005.
The nationwide manhunt for Tad Cummins continues and on Tuesday authorities said they suspect the former teacher and student could have left the country for Mexico.