to scrape, advantageous
戜 centers on scraping or cutting with a sharp tool, and by extension something that is advantageous or sharp-edged in a figurative sense.
戜 combines 戈 (halberd, weapon) with 呈 (display, present). The exact historical development is uncertain, but it likely relates to scraping or cutting with a sharp tool, possibly extended to advantageousness.
A halberd (戈) displayed (呈) for scraping and cutting: imagine a sharp blade presented for use, making it advantageous in battle.
For テツ, picture a sharp iron (鉄, てつ) blade scraping against metal: 鉄 -> テツ, and the scraping action reinforces the meaning.