Scales of training - Contact in canter
Contact is a result of (consequence) having achieved rhythm and suppleness first. The horse is working from the hind legs over his back into an equal contact.
This is created by keeping an elastic elbow that follows the movement of the horse, ensuring continual activity from the hind legs
In this video it is demonstrated well.
If the contact is not equal, heavier in one had than the other, make sure they take the contact in the rein they are dropping.
If necessary slightly flex them for a few strides and then straighten them again. Repeat as necessary.
If the horse loses balance and falls on the forehand the contact will become heavy.
When a horse is well balanced and in self-carriage, contact with the mouth is soft and even and your elbows follow the movement of the horses head.