The physical body is held onto in a more obvious way at a feeling level but there is also holding at the deepest levels with our self - identification with the body.
The sensations of the body range from the very gross sensations to the very subtle. At the gross level the easiest sensation to recognise is the force of gravity which bears down on the soft tissue and also influences the body fluids especially the blood.
When standing, it is impossible to relax the physical body completely unless the body is aligned correctly in the vertical plane with the force of gravity running through its centre. Even if the body is only leaning slightly there will be many muscles needed to counteract the gravitational effect which pushes the body into a more unstable state. A slight lean will always lead to further leaning and eventual physical disturbance as soon as you try to let go. So, at the beginning it is important to align the body vertically from the foot to the top of the head.
The weight should be centred in middle of the foot and neither forward, with too much pressure on the ball of the foot and the toes, nor too far back in the foot, with too much pressure on the heels. The alignment of the rest of the body from the base of the feet is achieved by deciding to practise with very little strength and at the same time becoming very mindful of the subtle muscular tensions which are needed to compensate for the misalignment. When these tensions become familiar it is possible then to find the correct alignment that will allow release of the original holding.