力 on the right is the strength radical, a stylised drawing of a flexed arm or plough. It locks the meaning down: 励 is to apply force on the spirit, to muscle someone toward action. Compare 努 (exert), 劲 (energy), 助 (help).
厉 supplies the reading lì directly with no shift. Its sense of being stern or whetting a blade also colors the meaning: to 励 is to spur and sharpen someone's resolve, push them to take a stricter stance.
