Bits with keys are designed for young horses. The horse fidgets with the keys in his mouth (by using his tongue) , and in result, it causes him to relax and soften. It is a good bit for decreasing the amount of times your horse fidgets, as well as helping to teaching a young horse to accept the bit.
Yes they can cause the horse to drool. But in dressage drooling or foaming at the mouth is a sign that the horse is soft and accepting the bit.
If you have any more questions or comments feel free to ask. :)