It just works (for code written to Lua 5.1; it's about 95% compatible with Lua 5.2, which changed how modules work) and can pretty much replace Lua (either the standalone executable, or linking it as a library). The "alien tech" probably comes from how it works, converting Lua to native code. I can't say how difficult to understand, since I haven't really looked into the internals, but from what I understand, Mike Pall created a new way of looking at assembly language as part of LuaJIT.

Thanks! It's also amazing how he did it with little resources compared to other projects.

It was a pun on Common Lisp (great language btw) - http://lisperati.com/logo.html But then after seeing LuaJIT2 I think there was something even more alien. It made me understand that I don't know programming very well after seeing what Mike Pall did there.