Around Keichō, the smith of the Shimosaka line was summoned to forge for Tokugawa Ieyasu and Hidetada, and as a result Ieyasu bestowed on him the character 康, whereupon he changed his name to Yasutsugu and was granted the right to cut the triple-hollyhock crest on his tang. The published sources record that the line originated in Ōmi, moved to under the patronage of Yūki Hideyasu, and thereafter served the shogunal house in alternating years between and , signing the long inscriptions that name the imported nanban-tetsu and of . The first generation, who re-tempered fire-damaged masterworks such as the Shishi Sadamune after the fall of Ōsaka, founded the house; this entry is read chiefly through the second generation, his legitimate heir Shimosaka Ichinojō, who took the tonsure as Kōetsu and died in Shōhō 3 (1646), and whose finest blades the published commentary holds difficult to tell from his father's.
The hand is read in the first. It is an mixed with that stands somewhat, in places flowing into , with thick , entering, and at times , the steel taking on the dark, blackish cast the published sources name as so-called steel. This dark, standing is the constant across his work, and the commentary returns to it as the school's signature, noting again and again that the forging "displays the characteristics of steel". Over it the temper is built on into which and are mixed, and entering, the strong with patches of coarse gathering unevenly and scattering into the broken (basake) appearance that is his most personal effect; and run long, forming a striped course, the tending subdued (), with along the back.
The is the surest single tell of the hand. It is most often a , strongly swept in and turning back long, the published sources reading on one that the temper "enters in , sweeps strongly in , and turns back long with a at the tip"; on the quieter pieces it shows the so-called flavor with the long, strong return. The rounded, swept with its long recurs across the corpus, the point at times pointed () or running in as , but the long swept return is constant, and the judges read it together with the dark steel as the smith's own. A second native register sits beside the manner, and the published sources call it his intrinsic style: a base, at times a medium , into which small connected are mixed, entering vigorously, the back generally hardened and the again subdued. On one such the commentary names this the very type of his proper hand, "the intrinsic workmanship of the second-generation Yasutsugu", the linked over a straight foundation, so that even where the temper quiets to the steel keeps the smith's name.
The register the line is most celebrated for is its -, the faithful copying of masterworks at which the published sources say both the first and second generations excelled. The corpus holds reduced copies of the great : an Ataka Sadamune carrying the gold-inlaid mark of the copy and a reducing the blade, and the only known second-generation copy of the Shishi Sadamune, whose burned original the first generation had re-tempered after the fall of Ōsaka. On these the forging tightens, the temper widens into mixed with and large and breaks into a large-pattern with and toward a feeling, the well adhered and the running almost as one sweep. On a the judges read the deep , the well-adhered and the and as an intent "aimed at the superior old masters", while observing that the standing and the subdued still betray the hand. Distinct again is the carving: the deep, forceful Kinai-bori executed with the engravers Kiuchi Chisō and Kinai Tomochika, the three-deity figures, , the dragon, and , by which the commentary even separates the generations, since the first generation's three deities are limited to Fudō Myōō with his two attendants while the second combines Jizō, Bishamonten and Monju.
What sets the second generation apart, in the judges' own framing, is how close he comes to the first while never quite reaching his scale. The commentary repeatedly measures him against his father: one it calls the work among his that "shows the style closest to the first generation", and a late it praises as a blade in which "a level of skill not inferior to the first generation is demonstrated to the fullest". The signature itself carries the distinction, the form of the character 継 differing from the first generation's, and the line is read not against the old it copied but by its own marks, the dark steel, the basake , the striped , the long swept return, and the deep Kinai carving.
Fujishiro grades him Jō-jō , and the Tōkō Taikan rates him at the upper end of the field. He carries no designations at the highest national tiers; his record runs instead through twenty-four blades at the rank and two designated Jūyō Bijutsuhin before the war, the latter recorded in the Yasutsugu Taikan and held in the Kurokawa Institute of Ancient Cultures. The provenance roll is and warrior: the Honda house, with ownership inscriptions of Honda Hida-no-kami and Honda Shichizaemon, several blades carrying gold-inlaid cutting-test inscriptions, one bearing the eccentric added line " hikashi," the smith's boast that even Mount stands low beside his sword. These are not blades that come to market often; with none held back at the highest protected tier yet most kept in long-standing collections, a signed Yasutsugu with the crest appears from time to time and with patience, the broad, dark, powerfully built work the commentary calls full of "force and rustic flavor" and well showing "the characteristics of work".