This bit is a fat, hollow-mouthed snaffle with holes through the mouthpiece.
When I first saw this bit, I have to admit that I had no idea what it was used for. I asked my readers to contact me if they knew anything about the bit and what it's used for.
A number of readers wrote in with more or less the same explanation:
"The whistle bit is used on horses that habitually windsuck. It's used while the horse is in the stall. How it works is that as the horse attempts to windsuck, the air is redirected through the holes in the bit, so the horse isn't able to effectively suck wind into the stomach."
Now, as food for thought here, another reader offered a different explanation of the bit's use and function:
"The bit as prevention of successful windsucking thing is a bizarre old wives' tale. Think about the anatomy involved for a moment--it would
be like saying you couldn't swallow or breathe through your mouth if you were eating a Lifesaver candy."
"The true purpose of the whistle bit is simply to encourage the horse to salivate, and to keep the bit moister in the horse's mouth by holding
saliva in the holes."