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.