The SRBs committed the shuttle to liftoff and ascent (to orbit) flight, without the possibility of launch or liftoff/ascent abort, until both motors had fully, and simultaneously, fulfilled their functions, consumed their propellants, were producing zero net reaction thrust and had been jettisoned (again simultaneously) by explosive jettisoning bolts from the remainder of the vehicle launch "stack" (shuttle w/engines; fuel/oxidizer tank).