That comment was supposedly put in the code for an intern who was still learning about the system. "You are not expected to understand this", as in, "this will not be on the test".
The comment was put there by Dennis Ritchie to state that the context switching code was hard to understand. It depended on the hardware of the PDP-11. He wrote that comment to indicate that it was complex, buggy, and that they didn't quite fully understand the code either.