I use the follow feature at times. Yes, it will slow all the way down. The gap is adjustable, maybe 3-4 distances. It will only speed up to the speed you set.
Example. Speed limit is 70. I set my cruise at 75. Eventually, I get within range of a slower car. My car slows down and maintains the gap I set. The guy in front of me takes off like a BOOH. My car speeds up until it reaches 75 and resumes my set speed.
If you are not paying attention, you will often slow down and not realize it..going miles slower than your usual. Once you change lanes, it will go back up until it 'sees' a new obstacle. It is useful in light to medium long distance highway travel. It leaves too much gap for heavy traffic, even at the closest setting. Like with any cruise setting, if you step on the accelerator, it will do as you ask...but if you take your foot off, it will brake to resume the 'gap'.