system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Cars have an “idle speed” where the engine still runs even without pressing the gas. What happens when you’re pressing the brake and completely still? How is the engine not harmed since it’s still running?
gpt
Automatic transmissions have a fluid coupling between the engine and transmission that allows the engine to run at low RPM without transmitting much torque to the transmission.  Imagine a propeller spinning, making the fluid spin, then the fluid makes another propeller spin.  

Manual transmissions disengage the clutch so there's no connection between the engine and transmission when you push the clutch pedal in.