obstruct, impede, stop, resist
擋 centers on blocking or holding back movement: obstructing, impeding, stopping, or resisting something that tries to pass.
擋 combines 扌 (hand) with 當, which likely contributes the sound and the idea of facing or blocking. The hand radical suggests physically stopping or resisting something.
The hand 扌 beside 當 suggests a hand held up to block or stop something. Picture a hand pushing against an obstacle to obstruct its path.
For トウ, imagine a hand blocking a toe from moving forward: toe -> トウ, and the hand stops it.