From the pictures and illustrations I have seen, The BDS is basically a UDS. They are vertical cookers and the smoke and the heat pass over the food as it exits the cooker.
If this is the case, there would be no need for reverse flow.
Maybe the BDS you have seen is different, but reverse flow are used on horizontal smokers so that the heat and smoke are somewhat evenly distributed across the food.
Humphrey's DownEast Beast W/BBQ Guru