In an automatic transmission, as long as you’re in Drive the transmission is in gear. There’s a special fluid coupling (called a “torque converter”) between the engine and transmission that lets the engine keep spinning when you stop the car with the brakes, but as soon as you release the brakes the torque coming through the torque converter is enough to start you rolling.
Two things:
1. I don’t know the exact details of why this happens, but if a car’s engine turns any slower than about 1,000 RPM, it will stall. So, the car is designed so that the engine is always turning at at least 1,000 RPM even when you’re not touching the accelerator.
2. A car with an “automatic” transmissions always has its engine powering its wheels as long as you’re not braking. So, that leisurely little 1,000 RPM makes the car move without any accelerator input.
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