to hurry, to go to and fro
遄 centers on rapid movement: hurrying somewhere or going back and forth quickly.
遄 combines the movement radical 辶 with 耑, which likely contributes the sound and may relate to the idea of swiftness. The exact historical development is uncertain.
The movement radical 辶 wraps around 耑, suggesting a person rushing along a path. Picture someone dashing back and forth so quickly that the inner shape becomes a blur.
For セン, imagine a person hurrying to send a message: send -> セン, and the movement radical shows them rushing to deliver it.