Back in 1970, when I began riding seriously, my BHI (graduate of Morven Park) instructor introduced me to the concept of behind the bit quite early in my riding career. What she said:
1) DON'T DO IT
2) It was a sign of bad horsemanship
3) Good riders NEVER rode behind the bit, at the top levels it JUST WASN'T DONE.
Almost 40 years later, everywhere I look, horses ridden behind the bit are EVERYWHERE, in all disciplines, winning top prizes, including medals at the Olympics. What happened?
I have a rather extensive horse library, including at least a quarter of the dressage masters. They ALL speak out against the horse going behind the bit. The Duke of Newcastle, (though how could he avoid it?) and De la Gueriniere, the foundation of modern dressage, speak out against riding behind the bit. Apparantly those horrible looking curb bits used by these riders limited the amount that the horse's heads could get behind the bit. There were chains joining the ends of the shanks, and the horse brought these against its breast, depriving the rider of all bit control. De la Gueriniere specifically speaks out against short shanked curbs (like we use today) because the horse could not protect itself from the rider's bad hands. In fact he considered ALL short shanked curbs as more severe than his long shanked bits, too severe to used in 'dressing" horses. The Duke of Newcastle and de la Gueriniere never practiced anything like Rollkur when using a bit, because they could not as their bits' long shanks prevented excessive overbending.
Then the bits changed. The warnings of the dressage masters of riding behind the bit become more dire and specific. RIDING BEHIND THE BIT IS ALWAYS DISCOURAGED. Why? The concensus seems to be that when the horse is behind the bit, he shifts the weight from his hindquarters to his forehand, contrary to all the rules of classical dressage. Since one of the keys to classical dressage is the movement of the horse's weight from him forehand to his hindquarters, a horses is not collected in any way when moving behind the bit.
How do you tell if a horse, or the horse you are riding is behind the bit? Look at the top of his neck. Ideally, the horse's neck should rise from his withers/shoulders in a smooth, gradual, and somewhat vertical arch up to his poll, with the horse's poll being the highest point of the horse. This is perfect for collection. Then there is the slightly less desirable neck, where the neck rises up to around his 3rd & 4th vertebrae, where the top of the neck levels off, flat and horizontal to the poll. A less perfect collection can be obtained from a horse moving with this neck. ANY TIME the top of the neck, from the 3rd vertebrae to the poll slopes downward, the horse is behind the bit and shifting its weight to the forehand.
This knowledge was taught to all serious English hunt seat riders when I started riding.
Look at the pictures and videos of riders today. A LOT OF HORSES RIDDEN ON CONTACT ARE BEHIND THE BIT on the flat.
Please note that this consensus against riding behind the bit often has to be interpreted from the writer's description of the movement of the horse, which rather than being conveniently spelled out in clear language in one place, is described murkily in several unindexed passages.
Enjoy your ride.