At the start of a guided meditation, the teacher usually gives some instruction about posture. The goal is usually to get the student into a position where they can sit still and comfortably, while also remaining alert.
To achieve this, the legs and hips are arranged as a stable base for the body. Then the torso and spine are upright, with the arms and shoulders relaxed, and then the head is held in a position of balance.
If we looks at these qualities of posture, they are also the qualities of mind that we want in meditation: stable, alert, relaxed and balanced.
Throughout the session of meditation, the mind sometimes notices the body. If these qualities are present they act as prompts and reminders for the mind. Specially so if we have set them up deliberately.
First, when we are setting up the posture, the mind is engaged. Then, with that posture established, the mind notices the qualities in the body and, to some degree, is influenced by them.
The posture of the body acts as a background anchor to all of our sitting meditations.
Pierre the Meditator 