This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 勈 is uncertain. It visually combines 甬 and 力, but the original semantic or phonetic role of these components is unclear.
With 甬 (a pathway or tube) and 力 (strength), imagine forcing strength through a narrow passage, but the exact meaning is unclear.
For ヨウ, imagine a strong person named Yo pushing through a narrow tube: Yo -> ヨウ, and the force 力 moves through the passage 甬.