How could a society be jump-started?

A "jump-start" is something you do to revive a dead battery. So by using this term, you are implying that technologically speaking the middle ages were at a stand-still ... that they were "dead". This is simply not the case.

Technology continued apace throughout the middle ages and in fact accelerated. The late Roman era had been stagnant. The "fall" of Rome (really more accurately described as simply the end of Roman political hegemony) was the "jump start" that Europe needed. Once free of imperial bureaucracy and old ways of thinking that idealized the past, scholars, craftsmen, etc. were free to explore new ways of thinking and looking at things.

Some of the technological advances of the middle ages, that Rome did not have, included shoed horses, stirrups, contoured saddles, collars (instead of yokes) for horse - all of which allowed better utilization of a horse's strength. This improved farming, commerce and the military.

Waterwheels and windmills also developed in the middle ages which allowed for easier irrigation, drainage, grinding of grain etc. Vast tracts of areas like France that had been untamed wilderness were brought under cultivation in this era. Likewise, much of the Lowlands (Holland, Belgium, parts of Denmark and France) was reclaimed from the sea by use of this new technology in the middle ages.

Medieval craftsmen also learned better ways of working steel, processing cloth, making paper, ink, etc., etc. Farmers developed ingenious ways of re-fertilizing land by using the dregs of fish ponds (a process we have mostly negelcted in our modern reliance on chemical fertilizers). And scholars, far from being the superstitious wags that the "Enlightment" philosophes would have had us believe, actual developed the scientific method that made the "Scientific Revolution" of the 16th & 17th centuries possible.

You should note that the ancient world did not have compasses. They were first used in China, but there they were mostly a novelty, used for proper alignment of Feng Shui and things like that. However, when the compass came to Europe, it spread like wildfire on a dry prairie and was almost university adopted by sailors, merchants, mapmakers and the military in the span of less than 10 years. This demonstrates the adaptibilty of the medieval European mind to new ideas.

So what do I think? First of all that your conception of the middle ages is a little outdated. Second if you wanted to stimulate _development_, the steam engine would be a winner, but they would have to have machines that could be steam driven. This is why introducing steam power to Rome would have been useless, but introducing steam to a society that was already using water and wind power (waterwheels and windmills) to run machinery does represent significant advancement.