I think it is the LImeWire engine. Major differences I see (other than ads) is that Swapnut users are required to share files where LimeWire allows you to not share or set bandwidth to 0%. If by doing this Swapnut keeps me from having multiple downloads showing 0.00 Kbps transfers, then that in itself will be worth it. Also, I only see SwapNut users on my searches. Not sure if this is a good or bad thing, although almost every download I have tried has been successful.