What is this "roll off the line" statement?

Is he saying that he gets his car going 4-5 mph and, takes his foot off of the accelerator? Then of course the transmission will shift into second gear, even at 4-5 mph. That is it's job! And then when you push down on the accelerator again, the transmission will have to change gears at that point, which would be back into 1st gear. You could also do that manually by moving the indicator from "D" back to "1".

But who drives like that?