bottom, base, slap, bang, all of a sudden
礑 centers on a sudden, forceful impact or the solid base that receives it: a slap, a bang, or the bottom of something. The 'all of a sudden' sense extends naturally from the idea of a sharp, abrupt strike.
礑 combines 石 (stone) with 當, which likely contributes the sound and the idea of striking or hitting a solid surface. The exact historical development is uncertain.
A stone (石) on the left and 當 on the right: imagine a stone hitting a hard surface with a sudden slap or bang, or forming the solid bottom base of something.
For トウ, picture a toe (トウ) stubbing against a stone base with a sudden bang: toe -> トウ, and the impact is sharp and abrupt.