to draw a bow to the full
彍 centers on the action of pulling a bowstring back to its maximum extent, ready to release.
彍 combines 弓 (bow) with 廣, which likely contributes the sound and the idea of extending or widening, together expressing drawing a bow to its full extent.
The left side is a bow 弓, and the right side 廣 suggests something broad or extended. Picture pulling the bowstring back until the bow is fully stretched wide.
For カク, imagine the bowstring pulled back to its full extent with a sharp 'kak' sound as it locks into place: kak -> カク.