This is the Chifney bit,
As said, it is not for riding but for leading a difficult horse.
A horse rears for many reasons. One can be to frighten the rider, to evade work, fear, bad hands or pain.
I have never known this bit to stop a horse rearing.
One of the worse accidents I witnessed was when some idiot of a girl had a Chifney in her horses mouth and attached a martingale to the lead ring. The horse reared, felt the pressure on the lower jaw and fought against it going over on top of the rider.