Most racing gearboxes have dog-engage, straight cut gears. Now, I don't know the specific technical aspects of how they work, but what that basically means is you can get them into gear without disenganging drive via a clutch or converter, or sychro. However, you will need to lift to upshift to lessen the old just enough to engage, though often today on EFI cars this is done by a shift-cut built into the engine ECU which momentairly cuts spark just enough for the shift to happen. On downshift, some drivers use the clutch and heel-and-toe the throttle and brake, whilst others while just bang it down a gear, giving it a rev. To downshift sans clutch you will need to rev and match revs.
Truckies don't use the clucth cause their gearboxes don't have synchro so they need to match revs to engage the next gear.
Faster, faster, faster, until the thrill of speed overcomes the fear of death...
– Hunter Thompson