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The point of a transmission is to allow variable torque, speed and power.
I assume you've ridden a bicycle with multiple gears (12 speed, 15 speed, 18 speed), and the principle is exactly the same.
f you try to start in 15th gear on a mountain bike, it's nearly impossible. The torque necessary to turn a large gear is much greater than the torque necessary to spin a small gear. On the flipside, if we want to go FAST, we are limited by the size of the gear. If the gear is small, then we have to spin the pedals impossibly fast in order to make the back wheel spin. As we select larger gears, the amount that we have to rotate the pedals to rotate the back wheel decreases, and we increase our efficiency.
As you pedal on a multispeed bicycle, your speed increases until you feel that you are pedaling "too much" to rotate the bicycle wheel. You shift up in a car for the same reason. The RPM of the engine is too high to rotate the wheels.
If you were going very quickly downhill on a bicycle, you have two options. You can either use the brakes to slow, or you can continue pedalling, shifting the bike into lower and lower gears until you begin to slow the bike down with the power of your legs against the pedal.
The same holds true for a car. If you want to decelerate quickly, disengage the clutch and press the brakes. If you want to decelerate slowly (and retain the stability of the car), use the "leg force" of the engine by shifting down into a lower gear. The decreased torque increases the engine speed, and the gear itself can be used to slow the car without altering the front/rear weight distribution. You'll really only ever do this if you're on slick surfaces and don't want to risk losing control from brake pressure.
And just like on a bike where you're slowing and then speeding back up, you downshift into a gear that feels comfortable but still allows you to pedal quickly when you speed up again.
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"I'm typing on a computer of science, which is being sent by science wires to a little science server where you can access it. I'm not typing on a computer of philosophy or religion or whatever other thing you think can be used to understand the universe because they're a poor substitute in the role of understanding the universe which exists independent from ourselves." - Willravel
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