Final answer:
When setting your tractor-trailer in motion, you can tell the friction point of your clutch has been reached when your RPMs decrease.
Step-by-step explanation:
When setting your tractor-trailer in motion, you can tell the friction point of your clutch has been reached when your RPMs decrease. This happens because the clutch engages and allows power to be transferred from the engine to the transmission. As the clutch engages, the friction between the clutch disc and the flywheel causes the RPMs to decrease.