open eyes, stare
睜 centers on the act of opening the eyes wide, often with intensity: staring or gazing fixedly.
睜 combines 目 (eye) with 爭, which likely contributes the sound and may suggest a sense of straining or opening wide. The exact historical development is uncertain.
The left side is an eye 目, and the right side 爭 looks like a hand holding a tool, as if prying the eye open. Picture someone forcing their eyes wide to stare.
For セイ, imagine staring so hard that you say 'say' something: 'say' -> セイ, and your eyes are wide open.