protrude, snap, break, dig
撅 centers on forceful, abrupt action: thrusting or sticking out, snapping or breaking something, and digging into the ground. The meanings share a sense of sudden, decisive movement.
撅 combines 扌 (hand) with 厥, which likely contributes the sound. The exact historical development is uncertain, but the character is associated with forceful hand actions.
The hand radical 扌 beside 厥 suggests a hand forcefully thrusting, snapping, or digging. Picture a hand breaking a stick or digging into the earth with a sudden motion.
For ケツ, imagine a hand snapping a stick with a sharp 'kets' sound: kets -> ケツ, matching the abrupt action of the kanji.