The first requirement for effective use of the riding aids is a balanced seat. Only when the rider is able to control their seat, can aids be given independently. A correct seat is also one that follows and support the movement of the horse without disturbing them.
Many riders make the mistake of sitting very straight wanting the aids to be at the right place. The vertical "shoulder-hip-heel" line is the basis of a correct dressage seat, nevertheless it is important for the rider to remain relaxed in order to be able to communicate correctly with the horse.
Using examples of unbalanced positions you will see why it is so important for the connection between the rider's hand and the horse's mouth to form one line and why the correct use of the weight aids is so important for your horse to move correctly.