shackles, irons, handcuffs, bonds
枷 centers on physical restraints that bind the body: shackles, irons, handcuffs, or bonds that limit movement.
枷 combines 木 (wood) with 加, which likely contributes the sound and the idea of adding or imposing something. Historically, it referred to wooden restraints such as a cangue or pillory.
A wooden 木 restraint that is added 加 to a person: imagine a heavy wooden collar locked around someone's neck, binding them in place.
For カ, picture a wooden shackle that clicks shut with a sharp 'ka' sound, like a lock snapping closed.