When the snaffle bit came into being, the maker stopped right there. It seemed perfectly suited for its job. However, it can lead a horse to push against the bit pressure, which puts his body in an unnatural position. When a horse leans on a bridle like that, his front foot goes too far out in front for each stride.
This causes his back to hollow, makes it hard for that foot to turn over correctly before the hind foot comes through, and makes it hard for him to do his job.
The Ported D has a stationary port, so it places pressure on the edges of the horse’s bars and provides tongue release. The horse easily understands and responds to this type of pressure instead of reacting to pressure he doesn't understand.
The entire setup is on the heavier side, so it gives more signal—there are no surprises to the horse with this bit.
I most often recommend the Ported D for starting colts, but it can be used on horses of all levels. When a horse is tight, it lets him take a breath and think about his job and relax.
“When a horse pushes and leans into pressure, his body goes in an unnatural position. The Ported D offers tongue release, a very kind port, and pressure that's universally understood.” —JE