In the old days, the idle was only controlled by a simple mechanical adjustment which lets through a fixed amount of air, which means the engine makes a roughly

In the old days, the idle was only controlled by a simple mechanical adjustment which lets through a fixed amount of air, which means the engine makes a roughly fixed amount of power. When you let out the clutch, the revs would drop a lot, each stroke would get more air so the torque would come up some, but it's fundamentally an open-loop system and didn't do a great job when tested like this. Newer cars have better engine computers which have made this a lot easier. In a modern engine the computer controls the idle air supply. It also can control the ignition timing, which provides very fast feedback. Between those, if you're careful feathering, the revs will drop a bit at first but the ECU will quickly bring on some more power to compensate.