While running, each change in position of Samus' cannon moves her forward by one pixel.

While mashing as fast as possible will create the most arm pumps and give the most benefit, even a relaxed arm pump will speed up several rooms by a significant number of frames.

That said, it is important to keep in mind the small amount of timesave relative to other aspects of the run -- it's best to avoid arm pumping if it has a chance of interfering with other controls. In this example, accidentally shooting a Super while holding Angle Up or Angle Down will likely result in bonking the door, and will cost much more time than arm pumping saved for that room.