Driving with the Allison Automatic Transmission

We have a 98 HR with the 5.9L Cummins and 6 speed Allison. During the 4000 miles we have driven it I have just kept it in D. It has performed fine. I have been curious about the economy mode but have been afraid of pushing the mode button since the manual says nothing about it. Can someone explain exactly how it works?
And I have used the exhaust brake quite a bit It surely helps keeping things under control. Especially on long grades.
Thanks in advance for any clarifications.
Keld