Well, as a disclaimer, I consider converters just short of VooDoo, but very simplified, imagine two fans facing one another, you turn one on, the other one turns in reaction to the movement from the first. Make it hydraulic instead of air, close the system to make it more efficient, and add a clutch that mechanically "locks" the two at some point, and that's how a stock converter works.
A perfomance converter for the most part plays with how efficient they are, at what speed the pressure will either have to give or the engine will stall (stall speed) and better internal parts.
Like I said, VOODOO, but the people I know who DO understand them say they're worth the money for a good one, only changing the stall speed on our motors is a whole different animal from what more traditional (read, mustangs, comaros other small block lighter weight vehicles) converter strategy would do. Somewhere there is an article about what to consider when you tweek your converter on a 5,000 truck compared to a regular car, the weight has a lot to do with it. I'll try to find it, so I can be coherent.
Hope this helps,