From the rules sheet:
The organizers of the race feel annoyed by the strange habit of the Camels to form stacks and carry each other along. To prevent this from now on, they introduce the Referee Camel.
The game is played as usual. At the beginning of each Leg, the player to the right of the player owning the Leg Starting Player marker places the Referee Camel onto any free space of the Race track.
When a Camel unit ends its movement on the space of the Referee Camel, all Camels of that unit are asked to "unstack" and to line up next to each other on that space. The Camels are "unstacked" from top to bottom and are lined up in the same order from left to right. (Therefore, the Camel on top is placed on the very left at the edge of the board and continues to be the leading camel of the former unit.)
Once all Camels of the unit have been "unstacked" (or a single Camel has landed on the space), the Referee Camel is removed from the Race track and placed next to the Game board. It enters the game again at the beginning of the next Leg (see above).
This small expansion can be added to the Camel Up base game as well as the expansion Supercup.