Shifting a Aux Behind an Auto?

I have a engineer friend that works for a utility in Cali. They had a large transformer delivered by Perkins and being a bit of truck/equipment nerd he hit up one of the drivers. Driver told him most of their trucks were Allison trans, which i knew already. What threw me for a loop as apparently some/most/all? of them also have an aux box and they shift the aux on the fly on heavy pulls. Maybe i'm being dense, but how do you shift an aux box without a clutch? Also, if the Perkins Macks at...

Shifting a Aux Behind an Auto?

Shifting a Aux Behind an Auto?