The most detail he gave on them was in World of Ptavs, where the human scientists had already figured out how to produce a modest field (something like 10^3 or 10^4 to 1), but then found an alien wearing a much more advanced field which they estimated at more like 10^16. They presumably reverse-engineered the alien’s suit offscreen, and all stasis fields in stories chronologically after that were the more advanced ones.