Can someone write up a brief explination of why the various solutions work?

I enjoyed the challenge of this, but now want to learn from the other solutions?

What is the purpose of knock(knock)()()? Why does it work? Why would I ever want to use that form of a function outside of this kata?

The solutions that rewrote the global variable success, what keeps us from using that on other kata?

What are the best solutions for debugging and tracing? I stumbled upon the wrapping solution which did not give me any information, but somehow called the function twice to solve it. I learned a lot from the solutions that printed out the machine code, where can I find more info on this type of solutions?