Horses usually try to compensate for the lack of strength in their back and haunches by seeking support from the rider's hands. So if your horse does this, you might want to consider adding some strength training to your horse's regime.
"And why do we ask the horse to do two track work? To get his legs under him, and to get him listening to the aids." ~ Kyra Kyrklund
"Suppleness means that the muscles contract and de-contract, and this has to go through the entire horse, it cannot be blocked in the middle of the back, and it also has to go through the rider." ~ Susanne Miesner
"The less we do on cross country, the easier we are on our horses. Learn how to be more still. Don't touch their mouth unless you need to. Plan your turns." ~ William fox Pitt
Impress your Dressage judges by showing a clear difference between riding a corner and part of a circle.