These muscles do two motions – twisting the upper body and leaning the upper body sideways, both with lower body locked solid.
This machine is for the first of these movements, but rather than lock the lower body, it locks the upper body – while twisting the lower part. The effect on the body is the same.
See the video example here.
A key point is to lock the upper body – and this is done by pressing the chest into the pads – then ensuring there is no movement across the chest during the exercise.
The seat has different positions depending on flexibility and range of movement, try to use the maximum position or build up to it over time.
It’s always one side at a time, don’t be tempted to go further than one-side-max to central and then return. Going further uses momentum, and detracts from the value of the exercise.
There is a kneeling version of this machine – which is actually more common in the UK – but it has the slight drawback that the knees can move on their pad, which can introduce a little reduction in range of movement.
This is exercise M39 in my ABC7 System.