straighten (an arrow)
擶 centers on the action of straightening an arrow, likely by hand, to make it fly true.
擶 combines 扌 (hand) with 箭 (arrow), suggesting the action of straightening an arrow by hand. The exact historical development is uncertain, but the component relationship is clear.
The hand radical 扌 beside 箭 (arrow) shows a hand working on an arrow to straighten it.
For セン, picture a skilled fletcher using a sen (a thin, straight metal rod) to straighten the arrow shaft: sen -> セン.