Bypass circuits were a technology employed in Constitution-class ships.
In 2266, James T. Kirk suggested the USS Enterprise's dire power shortage be solved by "switch[ing] to bypass circuits." The notion was not viable, but Montgomery Scott told his commanding officer that the circuits had all been burned out in the risky maneuver to extend the Enterprise's deflectors around Harry Mudd's beleaguered cargo vessel.
Though the precise functionality of the bypass circuits was unknown – other than the vague implication that they diverted power requests to some other source than lithium crystals – their absence was a severe crisis for the crew of the Enterprise. They immediately had to put in at the nearest source of lithium, and apparently gave no thought to repairing the bypass circuits. (TOS: "Mudd's Women")