Left hand radical - the side-form of 手. Anchors 捅 as a hand action: poking, stabbing, jabbing with a stick or finger. Same radical groups 捅 with the family of pointed-hand verbs: 推 to push, 拉 to pull, 抓 to grab, 戳 to jab, 捣 to pound.
Right side 甬 supplies the sound - yǒng drifted to tǒng, regular initial alternation, the same series producing 桶 bucket, 痛 pain, 通 to pass through. 甬 means 'a covered passage,' a handy memory hook: poking creates a passage through something.
