Truck transmissions don't have synchromesh gears which allow you to shift even if the input and output shafts are rotating at a different speed. For a truck, you need to match speed with what the engine and wheels are doing. Shifting for truckers is a two-step process. Hit the clutch and put the transmission in neutral. You then quickly tap the accelerator to match engine revs with what the wheels are doing, hit the clutch and put it in gear. You can use this method to see how smoothly you are shifting normally, and helps you learn how to rev match. The benefits of rev matching include smoother transitions between gears, especially on downshifts as well as reducing the wear and tear on the synchromesh gears themselves.
Heel-and-toe: One of the great advantages of a manual is that you can anticipate a corner or hill and shift into the correct gear before slowing down or hitting the gas. In racing conditions, this happens very quickly, along with the need to brake hard and getting back on the gas very quickly. So quickly in fact, the sequence of pushing the clutch, shifting to a lower gear, braking, then getting back on the gas as you release the clutch is impossible. Even if you could, the weight transfer forward while quickly downshifting would probably upset the balance of the car. This is where heel-and-toeing comes it. It means that while you are braking you need to operate the clutch and the gas to rev match as you downshift, resulting in a smooth transition that doesn't unsettle the car and provides the right gear for acceleration out of the corner. Three pedals and two feet, hmmm... The way it is done is the left foot is used for the clutch, while the right foot spans both the brake and gas pedal, classically with toe on the brake and heel on the gas. This technique requires quite a bit of experience, and practicing how to double-clutch first would help.