The primary aspect in this training exercise is that the CB runs/crosses in as screening player against the direction of the passing.
C passes a contra pass to B, who crosses behind A.
The CB will in this way get used to set a screening after the crossing.
C becomes the new center player and B becomes right back and so on...
The ball must be played all round before the CB starts a new cross.
The exercise can however also be done without the screening moment.