Thompson didn't want to write about the usual things that Turing award winners write about— in fact, according to him,1 he didn't want to write a paper at all. However, when he did finally write a paper (after putting it off for a year past the original deadline, see ~2:52:49 in the above video), he describes at a high level how the C compiler (itself a program written in C) parses and compiles C source code into machine code to set the stage for how the "Trojan horse" would be injected into the generated machine code while leaving the original input C source code seemingly unmodified.