strike, whip, slap
捶 centers on striking with a tool or hand: hitting, whipping, or slapping as a forceful action.
捶 combines 扌 (hand) with 垂, which likely contributes the sound and suggests a downward hanging or striking motion.
The hand radical 扌 beside 垂 (which looks like something hanging down) suggests a hand bringing something down forcefully: striking, whipping, or slapping.
For スイ, imagine a swift strike with a whip: swift -> スイ, and the hand delivers the blow.