One of my Dressage trainer’s most valuable mantras is:
“The inside hand acts like a handbrake on the inside hindleg“.
Of course we always want the inside hind to be stepping through as much as that horse is capable of at any given moment, to lift the forehand, engage the back, etc, so it makes no sense at all to inhibit it in the slightest!
Therefore, the most the inside hand is allowed to do is to HOLD, it must never ever PULL or block the power coming through from behind. A holding (but still elastic) contact, applied correctly, is enough.
The aim, as with all aids, should be to progressively lighten it as the horse understands and accepts more, and becomes stronger, more balanced, more flexible etc.
Another of his mantras is:
“The moment you take up a contact, your hands belong to the horse“… you must think FORWARD with the hand, never pulling or blocking.
A horse going sweetly forward to a nice elastic forward-thinking contact is a joy to ride.