pull, tie up, prick, stab
扎 centers on forceful, pointed actions: pulling or tying something tightly, or piercing with a sharp object. The meanings share a sense of applying pressure or penetration.
扎 combines 扌 (hand) with 乚, a component that may suggest a hook or piercing tool. The exact historical development is uncertain, but the character likely depicts a hand using a sharp or binding instrument.
The left hand 扌 holds a hook-like tool 乚. Imagine pulling a rope tight with the hook, or using it to prick and stab.
For サツ, picture a sharp hook that suddenly pricks your hand: the sharp surprise makes you say 'satsu!' as you pull away.
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