The flanking forces pinned down the enemy's main force.
Tips
grammar
住 as a result complement indicates 'firmly, securely, so it stays.' Compare: 拿住 (grab and hold), 抓住 (catch and hold), 记住 (remember firmly), 拖住 (drag to a halt). Without 住, 拖 just means 'to drag' without the sense of stopping someone.