open
闢 centers on the act of opening something that is closed or blocked, especially in a physical or metaphorical sense of clearing a path or making way.
闢 combines 門 (gate) with 辟, which likely contributes the sound and the idea of opening or breaking through. The gate suggests a barrier being opened.
The gate 門 is being pushed open by the inner component 辟, like a door swinging wide to clear a path.
For ヘキ, imagine a heavy gate that opens with a loud 'heck!' sound: heck -> ヘキ, as the barrier swings open.