The Jake brake had me confused for a while. When we did our test ride the salesman told me the switch with the paddle selected the level and the other switch was the on/off switch. The truck documentation said the opposite, but it also said the Cummins should have three braking speeds. I only had two.
Eventually I removed the switches and checked the part numbers to determine their correct operation. The paddle switch is indeed the on/off switch. The reason the other switch only had two levels is because the switch was hosed. I purchased a new switch on Ebay and...holy cow...what an improvement! Before I apparently only had two levels...none and low level. Now I have three braking levels...low, medium, and high. High level is a beast. It might be my imagination but the transmission seems to interact with the Jake brake now.