In 1982 "Ole Mo" arrived in our rule books. In it's original form, only airborne kicks and pass interceptions were included. Prior to that if a pass was intercepted or kick caught outside of the endzone, with the momentum of the player carried him back into his endzone, he would have to scamper back out or it would be a safety. The only chance for a touchback would be if the force of the pass or kick drove the player back into his endzone. Some officials would rule that the powerful force of the puny little ball drove the player back.
By adding "ole' mo'", the team in possession was not unfairly hit with a safety; nor was the opponents hit with a touchback if the ball was possessed inside the 5 but outside the goal line. The rule since has expanded to include other events such as fumbles, grounded kicks and the like. Things to remember about "ole' mo" :
(1) The player's momentum has to have brought him back into the EZ, not if he was running in the field of play and reversing into the EZ.
(2) Possession has to be gained inside the 5.
(3) A player juggling a pass/ fumble into the endzone hasn't yet gained possession= touchback.
(4) A player juggling a kick into the endzone kills the play as a kick has entered the EZ =touchback.
(5) Ole' mo' leaves the building if player fumbles and ball goes OOB or is recovered in the field of play.
Hope this helps.