This magic works via detection of a STATUS control character, Control-T, by the kernel tty driver, which then prints the load line via tty_info() on FreeBSD or ttyinfo_locked() on Mac OS X. It also sends a SIGINFO to the process, so it can run its own routine. See STATUS in the termios(4) man page.
~ Brendan Gregg, from Control T for TENEX
It is so freakin’ cool when I learn new Unix tricks. It’s like a maze of twisty passages, all alike; Except there’s… oh! A piece of candy! oh! A piece of candy! oh! A piece of candy.