My Article part 1
The bit is accepted by the horse as a result of driving the horses rear quarters more underneath himself which causes the pulsations from this forward reach to pass along the spinal column, through the riders driving seat to the poll. This will cause the forehand to rise and the horse adapts to the bit according to its conformation.
Long explanation and easy to do once you understand the mechanics involved.
The biggest problem most people have is that they ride the horse from front to back and all that does is stifle the forward reach of the rear legs. The end result is the horses balance goes off and the horse either leans on the bit or throws their head up. The rear legs in both cases travel behind the horse and it will either run to try to bring it's balance together or just play the "lazy" game.
The other biggest fault is to ride the horse forward but to the point that the horse is running into the bit and is balancing itself on the bit. In this case the rider has failed to ask the horse to balance on the outside rein. A properly trained dressage horse is "caught" between a driving inside leg a supportive outside leg and a soft light inside rein and a communicative and supporting outside rein. Without this the horse has all sorts of evasions such as balking or running out.
I teach all my students to ride the rein forward. To do this the horse is on the outside rein with a little more pressure than the inside rein. The inside foot will prevent the horse from falling in. The outside foot applies enough pressure to turn the forehand 1/4 the width of the horse towards the inside. This ensures correct contact with the outside rein, prevent the falling out of the horse and act as a counter to the inside foot. After the initial hold release ONE rein very slightly only as far as the next step the horse will take and resume contact afterwards. This might appear as a circular almost massaging motion of the hand. Upon resumption of contact a slight vibration of the rein will ensure the horse does not "sit" on the rein. This is repeated on the opposite rein. THIS IS NOT SAWING as proper contact MUST be attained by one rein before the release of the opposite rein is started. DO NOT RELEASE the outside rein to the horse when in training and in a corner or on a curve.
Very shortly you will find that the horse will maintain a headset according to the degree of collection requested with only the slightest touch of the rein. and you can test this by a full release of both reins for a step or two.