deep magic
[possibly from C. S. Lewis's "Narnia" books] An awesomely arcane technique central to a program or system, especially one neither generally published nor available to hackers at large (compare black art); one that could only have been composed by a true wizard. Compiler optimisation techniques and many aspects of OS design used to be deep magic; many techniques in cryptography, signal processing, graphics, and AI still are. Compare heavy wizardry. Especially found in comments of the form "Deep magic begins here.". Compare voodoo programming.