Keep On Pedal'n
One of the basics of riding in a paceline is to "always keep pedaling."
There are, of course, logical exceptions, such as slowing for a stop sign, or when not in the lead going downhill.
If you really want to piss off the rider behind you, you can be one of "those guys" who pedals furiously and then stops pedaling for a bit. This makes it difficult for the following rider to maintain a steady pace and is both physically and mentally tiring to them.
A major cause of on and off pedaling is following too closely. As a complex feedback loop, a paceline ebbs and flows a bit naturally. Most experienced riders know how to make the needed small speed adjustments without even thinking about it. However, if you're too close to the rider in front of you there is not room to make these adjustments without stopping pedaling!
Being too close also leads to momentary wheel overlaps. If you are overlapped when the rider in front needs to move over for an obstacle you're in for a quick trip to the pavement, and will likely take down following riders.
Best practice is to leave a gap of two to three feet between your front wheel and the next riders rear wheel. Adjust your speed as necessary to maintain whatever gap you are comfortable with; yo-yoing with your gap (even when pedaling) is about as bad as start and stop pedaling.