This is a classic individual defending drill used within Italian football to develop players capabilities to defend from behind. You might be asking why this trait is trained, but it provides a common situation in professional football and creates one of our biggest challenges as individuals when trying to defend.
The practice is very simple, when the coach blows the whistle the yellows look to run with possession of the ball through the gate at the end of the practice. However, there is a slight catch as the yellow moves, the red follows and toys to win possession back without fouling the yellow. The distance does need to be fairly considerable so that the red and yellow have the opportunity to compete physically for the ball. 

Making Contact

Something that doesn’t receive enough coaching or acceptance is the need to make contact with opponent when defending, especially when defending from behind. We can see two example on how the outcome might look in the examples above. In the top example, we can see how the red is looking to come in from the side, this is the correct approach to defending from behind. Importantly the red will be making contact with the side of the opponent, almost holding them to ensure that they can not escape the 1v1 moment.
Secondly they are trying to either get their body between the player and the ball to win possession back without the need for a tackle. Or secondly they might be looking to keep contact with the opponent to prevent them getting away, before initiating a slide tackle as the opponent is about to take a touch. This timing is vital to ensure that the tackle blocks the ball moving forward, this prevents the opponent winning a free kick and kicking the ball ahead before the tackle comes in from the red. 

Timing a tackle, this is an important skill for defensive players, can we encourage players to make clean contact and the correct moments. 

The perfect session for centre-backs to practice their timing when tackling and their defending when running towards their own goal.