look down, diligent
俛 centers on directing one's gaze or effort downward: looking down physically, or applying oneself diligently as if bending one's head to work.
俛 combines 亻 (person) with 免, which likely contributes the sound and may relate to bending or lowering. The exact historical development is uncertain.
A person 亻 beside 免, which looks like a bent figure with a head and legs. Imagine someone bowing their head to look down, or diligently working with their head lowered.
For フ, picture a person looking down at their feet, making a soft "fu" sound as they exhale while bending: fu -> フ.