put forth effort, strive forward
勱 centers on exerting oneself to move forward: putting forth effort and striving ahead with determination.
勱 combines 萬 (ten thousand, myriad) with 力 (strength, effort), suggesting the idea of exerting great effort or striving with the force of ten thousand.
The left side 萬 means ten thousand, and the right side 力 means strength. Picture putting forth the strength of ten thousand people to strive forward.
For バイ, imagine buying (買う, かう -> バイ) the strength of ten thousand to strive forward: buy -> バイ, and you purchase immense effort.