You can split downshifting as well as upshifting.
Doing line haul, you really won't find the need too often for the splitter.
The 13 speed is based off a 9 speed that you can split the top gears on. (5 low range and 4 high range)
An 18 speed is a 9 speed you split both the low and high side.
It's not necessary to split gears, if you are on flat ground, or pulling empty or a light load.
The shift pattern is close together, which makes this tranny easy to get use to.
As you get use to your tranny, some guys will float 60-80% of the gears, and do what's called skip shifting.
I skip shift, but rarely float. Main thing is, you want to shift smoothly, and you will want to grab gears as little as possible.
By that I mean, a good driver anticipates what is ahead and what is needed, and makes his decisions as conditions dictate,
whether that be weather, steep grades, traffic, how much you are hauling, etc.
Example; I'll speed up to tackle a hill, if no traffic is impeding me, in order to not have to downshift.
Here is an eaton fuller site, watch the movies on the rightside margin. (helpful advice)
Super 13