The state in which a computer is said to be when it rapidly alternates several times between being up and being down. Interestingly (and perhaps not by coincidence), many hardware vendors give out free yoyos at Usenix exhibits.
Sun Microsystems gave out logoised yoyos at SIGPLAN '88. Tourists staying at one of Atlanta's most respectable hotels were subsequently treated to the sight of 200 of the country's top computer scientists testing yo-yo algorithms in the lobby.
[Is it "yoyo" or "yo-yo"?]