Eazy D pretty much sums it up! The Rodimus Prime cab is essentially 2 parts that flip out from under the trailer, locking over the back end & sides of the Hot Rod mode to give it a new frontend look.
Actually, both vehicles look like they might attract criticism for being 'shellformers', but that's not a concern of mine... for such beautiful looking modes (all of them!)
Here's a good pic of the legs extending (plus face swap + wing reposition). They've been a bit tricky with the photo zoom though, to make RP look bigger than he actually is. In reality i'd guess it's about .5-1cm increase in height.
Many more high quality pics here!