Driving in the "drive" position, autoshifts shift up and down through the gears automatically as the truck speeds up and slows down. The driver can select any gear, up to, and including 5th, when first starting off.
The gear selected when starting off is the gear the transmission will downshift to at each subsequent stop unless the driver selects a different gear.
Driving in the "hold" poistion, the driver can shift when he/she chooses instead of letting the computers decide when to shift. When the driver selects a gear in the hold position, the transmission will remain in that gear until the driver selects a different gear, or the truck comes to a stop. When the truck comes to a stop, the transmission automatically returns to the gear the driver selected when first starting off.
The driver pushes a button on the "T" shift handle with his thumb to change gears while driving in the "hold" position. This gives the driver shifting control and makes driving more fun IMO
Hope that's not too confusing.