bit (horse)
轡 refers specifically to a horse's bit, the metal mouthpiece used with reins to control a horse.
轡 combines 車 (vehicle/cart) with 口 (mouth) and 糸 (thread/string), suggesting a mouthpiece connected by reins to a cart. The exact historical development is uncertain, but the components fit the meaning of a horse's bit.
The left side shows a cart (車) with threads (糸) leading to a mouth (口) on the right. Picture the reins running from the cart to the metal bit in the horse's mouth.
For ヒ, imagine the horse heaving against the bit: heave -> ヒ, as the bit pulls in its mouth.