This is an important skill for riders to learn early on and is covered in my EBook 'The Beginner Riders Handbook. (The link can be found on our website under Free Online Training) Our bodies have to be soft and supple while riding yet still be able to hold ourselves in the correct position. Often riders that are working on their position tend to over do it and become stiff and rigid in their body and arms which becomes a hindrance to our communication and also uncomfortable for the horse.
This takes practice but after a while you won't have to think about it, the subconscious part of your brain along with muscle memory will take over and do it naturally for you.
You can find a great video that explains this really well on our YouTube channel and our Facebook page.
Happy Riding and have a magical day!