Turns out there are two ways a jpeg can be saved.
Often times, jpegs are baseline images which means they load from top to bottom, revealing a little bit of the image at a time.
Alternatively, progressive jpegs show the entire image up front (at a reduced quality) and progressively load the full quality image.
On a webpage this means you'll get faster load times and can avoid the ugly experience of seeing full quality chunks of the image as it loads.
If you happen to be using Paperclip to do image upload and processing on the server-side you can do something like:
Where "-interlace Plane" creates a progressive jpeg instead of the default baseline jpeg.