Coast does exactly what you think. It will just let the motors move without any power being applied.
The real difference is between brake and hold. Hold fights to get back to position, while brake just resists movement in general.
STORYTIME!!! When one of my kids was making their 6 bar…
First, they used Brake. When they lifted the 6 bar up and applied Brake, the 6 bar slowly lowered itself. The motors resisted the movement but did not bring it back to position.
Then they tried Hold. This did exactly what they wanted, keeping the lift at the same position once up. Awesome!
Finally, their build wasn’t that good… And the gear on the right side would slip sometimes. Unfortunately, this would make the lift lean to one side. To fix it, they made one of the buttons apply Coast to the motors. The lift would drop and hit the base, straightening the lift out. Then they would go back to driving the robot.
So, they managed to use all three modes, and two were in their code at the end of the day. Next time they should just build better botz.