Breaking bits with keys are usually used as a starting for young horses to encourage mouthing and chewing.
The keys, are movable and this encourages them to accept the bit in the mouth.
Long cheek pieces prevent the bit from pulling through the horse’s mouth.
The bit can be used with keepers that attach to the bridle and keep the cheeks more or less vertical.
Thanks to the cheeks the bit lies evenly in a horse's mouth.