Well, flash nosebands are used a lot in racing and cross-country, to help keep the bit in the horse's mouth and keep it from dropping the bit while it's racing along. I'm not sure how it helps collect the horse as it can't work in the same way as the curb - unless it can put pressure on the poll, which I don't think it can? If anyone knows, please enlighten me :)
I know the curb is only used lightly and briefly, but I still wonder why it's allowed, given the principles behind riding in a snaffle ^^ It's a curious thing.
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