Even manual transmission cars these days have what is known as synchromesh, which facilitates the engaging of the gears in a downshift. The problem is that one end of the transmission is connected directly to the car's engine and the other end is connected directly to the drive shaft, which is moving at a speed directly proportional to the speed of the drive wheels. The gears in between aren't necessarily going to want to mesh with each other when those speeds are mismatched as they will be when you change gears. Before the days of synchromesh, there was a technique called double clutching that was used, mostly in downshifts, to match the speed of the gears to each other when making the shift. It involved depressing the clutch pedal, shifting into neutral, lifting your foot off the clutch pedal, revving the engine to the speed it would be going at the speed you are travelling in the gear you are shifting down to, depressing the clutch pedal, and completing the shift into the new, lower gear. It sounds terribly complicated from the description but it becomes second nature once you get the feel for it. My first vehicle was a 1950 Ford F-1 pickup truck with no synchromesh. I loved driving it. But I learned to double clutch on a 1939 GMC wrecker at my job at the sawmill. To this day, I will double clutch when downshifting on a manual transmission car.