The best dressage people don’t use devices over training. And part of correct contact is that it has to be light, and is created by the horse always stretching lightly down and out, so that the contact is created by the horse, not by the rider, who merely indicates where the horse’s head should be, with correct light contact, by either allowing the reins to slide through his/her hands until the horse has stretched as far as the rider wants, or by closing the fingers enough to stop the reins sliding through, which tells the horse when to stop pulling the reins through the riders fingers. When done right, the contact is there, but feather-light, created by the horse and rider communicating in harmony, so that to the rider it feels like he/she is riding with his/her fingertips.
The second the rider demands a frame by using devices, the horse is punished instead of rewarded, and actually ends up using & strengthening the wrong neck muscles to resist the forced contact, and one ends up with what is referred to as an upside-down neck.
If you keep in mind that is must always be the horse who creates the contact by stretching to the bit, where ever it is put by the rider, and the rider who indicates only where the horse’s head should be. In other words, the rider controls where he/she wants the bit to be, but the horse creates the contact by accepting and working with the length of rein (and therefor the bit position) the horse is given by the rider.
This sounds so easy in theory but it takes a subtlety and finesse on the rider’s part that is much harder than throwing a device on the horse to force the horse’s head to be in a specific place. Whenever we riders force a horse to do something, he will tend to resist. This is why it is so important that the horse choose to stretch to the bit, and choose to continue to stretch slightly to the bit so that the horse stays at the end of the reins.