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If the rider's hand is always calm, steady and firm, the horse will get confidence in our hand. When the contact gets lost the horse will look for the rider's hand. This is needed when we want to ride the horse on a long rein in our warming up period. This is certainly necessary for the lengthening of the stride.

In the lengthening we push our hands a little forward, the horse reaches for our hand and lengthens his frame. This enables him to free and open the shoulders.
There is a direct relation between the bit and the shoulder in the negative as well as the positive sense. So on the other hand, when we have to get tougher from time to time, the horse with confidence in our hand, will accept this stronger contact, without throwing the head about, jamming up and becoming stiff gaited.

To conclude, I would like to say that in my opinion, the horse finds mental and physical support in the contact. He will be looking as much for us as we are looking for him when the contact gets lost. Some mouths are harder than others, so don't be afraid of contact, as long as the hand of the rider is allowing.