to tread on, to stamp, to walk
蹹 centers on the action of pressing the foot down: treading on something, stamping firmly, or walking with deliberate steps.
蹹 combines 足 (foot) with 翕, which likely contributes the sound and may relate to closing or gathering, but the exact historical development is uncertain.
The left side is a foot 足, and the right side 翕 looks like a closed wing over a mouth. Imagine a foot stamping down on a wing, treading firmly as it walks.
For トウ, picture a toe pressing down hard: toe -> トウ, and the foot stamps and treads with each step.