“Why do Dressage riders hold such tight contact?” is a relatively common question from riders watching a dressage test or exhibition the first time. What these people don't realize, is although the reins appear to be very tight, just because you don't see slack in the reins doesn't mean the contact isn't soft. A rider at upper levels of dressage should have soft enough hands to be able to give and take contact in invisible fractions of pressure. Remember that if a horse's head is steady, a half inch forward movement of the hands gives the same amount of release as a foot of slack in the reins.
Keep in mind that dressage riders do not use tight reins to "pull the horse's head back and hold it there", the horses are trained to hold bring their necks up and arch their backs and just let their head relax off the poll. The contact on their mouth is (should) actually be very light.- It must be light in order for the horse to perform so precisely with such invisible aids.