Many daily activities require the ability to sit erect on a bench and maintain the trunk erect and steady when using the upper limbs. 

Here I use the term sitting independently to refer to the ability to sit on a raised surface with the hips and knees flexed to about 90 degrees, maintain the trunk erect and steady when moving the upper limbs to engage in hand tasks, including reaching in different directions, and to turn and extend the head to look in different directions. 

Activities that require sustained sitting with at least some degree of trunk stability and extension include getting dressed, eating meals, using the toilet, transitions from sitting to standing, and using the hands for interacting with objects. 

Training sitting by adapting the environment to change task demands 

If a child is not able to maintain the trunk erect and steady when sitting on a bench  or has difficulty maintaining balance, external support can be provided to the trunk.

I use weighted boxes positioned behind and to the side of the child to provide this support. 

The height of the boxes can be adjusted to provide just enough support to allow child to maintain an erect trunk.  

Initially the child may need to use hand support to maintain an erect posture, but with practice they learn to sit erect and lreach in different directions. 

Initially the boxes are positioned in close contact with the child's pelvis or trunk, but as the child's ability to sit erect improves the boxes are moved further away provide increased challenge for balance. 

Why use boxes instead of manual support from a therapist or caregiver?
Boxes provide a reliable degree of support and also provide surfaces that allow them to use upper limb support to assist with trunk extension and balance.  The child learns to use the support and opportunities provided by physical environment to engage in activities that interest them. The child is able to act independently in this environment. 

How does this differ from manual assistance?

When a therapist (or caregiver) provides the support the child needs to maintain a posture, the child and therapist form a unit. 

The therapist automatically provides and adapts the external support the child needs to perform an action or task. Therapists are really good at anticipating the amount and direction of support the child needs.

Think about carrying a  baby - they will turn the head to watch the world go buy. Each time they do this the caregiver adjusts to support they provide to keep the baby's trunk steady. 

The child is learning how to sit erect when manually supported. This is a very different experience from sitting with box support which provides an environment that requires independent action to maintain the position.