pick up, gather up, hold in lap
擷 centers on gathering or collecting things by hand: picking up items, gathering them together, or holding them in one's lap.
擷 combines 扌 (hand) with 頡, which likely contributes the sound and may relate to the idea of gathering or holding. The exact historical development is uncertain.
The hand radical 扌 beside 頡 suggests using your hands to pick up and gather things, like scooping items into your lap.
For ケツ, imagine gathering up ketchup packets with your hands: ketchup -> ケツ, and you hold them in your lap.