step on, tread on, lose energy
蹬 centers on the action of pressing down with the foot: stepping on, treading on, or pushing off. The sense of losing energy likely extends from the idea of stomping or stamping down forcefully, as if expending energy in a heavy step.
蹬 combines 足 (foot) with 登, which likely contributes the sound and the idea of ascending or stepping up. The character thus suggests a foot action related to stepping or pressing.
The left side is 足 (foot), and the right side is 登 (climb). Picture a foot stepping firmly onto a stair to climb up: that stepping action is 蹬.
For トウ, imagine stepping onto a tall tower (タワー -> トウ) with your foot. The tower cues the reading トウ while you tread on the first step.