The "Wujing Zongyao", written during the Song dynasty, notes that during the Tang period, crossbows were not used to their full effectiveness due to the fear of cavalry charges. The author's solution was to drill the soldiers to the point where rather than hide behind shieldbearers upon the approach of enemy soldier, they would "plant the feet like a firm mountain, and, unmoving at the front of the battle arrays, shoot thickly to the middle the enemy, and none among them will not fall down dead." The Song volley fire formation was described thus: "Those in the center of the formation should load while those on the outside of the formation should shoot, and when enemy gets close, then they should shelter themselves with small shields side shields, 旁牌, each taking turns and returning, so that those who are loading are within the formation. In this way the crossbows will not cease sounding." In addition to the Tang formation, the Song illustration also added a new label to the middle line of crossbowmen between the firing and reloading lines, known as the "advancing crossbows." Both Tang and Song manuals also made aware to the reader that "the accumulated arrows should be shot in a stream, which means that in front of them there must be no standing troops, and across them no horizontal formations."