I found this: A butterfly bit is a bit in the shape of a butterfly which sits (on either side) outside the mouth to allow the rider to exert more pressure on one side of the mouth or the other to keep the horse travelling more tractably. From here: Just Racing
And this: Our favorite bit for draft horses is the Butterfly Bit
. This bit is made of stainless steel and it has 3 large loops on each side of the mouthpiece. By choosing different loops you can adjust the amount of pressure applied to the mouth. They are used for riding and driving. From here: http://www.saddlesandharness.com/page/2/
Here are some more bits, but no good explanations: http://www.bitsnmore.com/reinsmangagdrawbits4.htm
This has a little more info: • Driving Butterfly bits
The Butterfly bit’s conception is close to the Gag bit’s, but its effect can be more severe as it has an additional ring on each side. Like for all bits with curb chain that have a leverage effect, the lowest you fasten the reins, the strongest the action. Traditionally, this bit is more often used with a breastplate harness than with a collar harness.