do you even manual transmission
rev match
shifting at the point where the transmission would've set the gear, you shift without use of the clutch, as an example, in my camry if you're rolling in neutral at 40mph exactly, and you rev the engine to somewhere between 1900 and 2000 rpm the shifter will just slide into 4th gear naturally because that's where the trans would've set the rpm at 40 mph anyway
engine brake: shifting into a lower gear at a speed where the transmission would set it at high revs, (between 3000 and redline) the physical force of the engine not wanting to go up in revs that quickly causes your car lose speed, its basically inertia applied for braking, you use this in racing to preserve your brake pads, if you went pure brake pads in a race, you'd surely have a brake fire (which actually happened to my dad once)