Slip shifting Maxforce 13 with automatic engine retarder.

I was looking for another semi tractor. I took a Navastar with a Maxforce 13 for a drive. Every time I took my foot out of the fuel, the engine retarder came on, down to about 1400 rpm. There was no switch to allow the retarder to be turned off. The retarder made it a pita to shift, as you had to bring the rpm back up to match the truck speed. I didn't buy the truck, but am wondering how you are supposed to shift such a combination.