Under a steady axial pull, an ordinary bolt is weakest at the root of its threads, because that section has a smaller cross-sectional area than the plain shank. But under a sudden shock or impact load, what matters is not just the peak stress but how much energy the bolt can soak up before it yields, and that energy capacity depends on the stressed volume of material as well as the stress level. If the shank is turned down so that its diameter equals the core (root) diameter of the thread, then every section along the bolt's length carries practically the same stress under load, instead of the stress jumping up sharply at the thread root. With no single weak section, the bolt can stretch more uniformly and absorb far more impact energy before failing. This is why such a bolt, with shank diameter reduced to the thread core diameter, is called a bolt of uniform strength, option (1).