This is a good defensive drill that involve all your backs (only touch). To start, have your backs set up in their usual defensive set up for a scrum. In this drill they are only allowed to jog and must use constant communication. Have your subs run against them using the whole width of the pitch in order for your team to work on their communication in defence, drift defence and your back three can work on their positioning skills when the attackers are switching the direction of play. 

Encourage your players to remain in a straight line and use constant communication. The main aim would be for no line breaks!