The beauty of pilates, especially on the machines is that each and every exercise can be modified. While some people may need adjustments to simplify an activity, others may need modifications in order to challenge themselves further.
Kneeling on a machine can be quite challenging, as you can see in the above picture. It requires a great deal of core strength and stability to be upright on the machine and imposing upper body movement on a steady base of support. This activity would most often be performed at an intermediate level of training.
For those who are not ready to kneel or find the postition uncomfortable, there are several simple modifications they can make and still receive maximum benefit from the exercise. Often clients will either sit on the edge of the reformer with their legs dangling over the side, or they will sit on top of the long box with their feet planted on the reformer.
In all of these scenarios, the essence of the exercise can be achieved but the approach will vary based on the specific needs of the individual. To make a very long story short, everyone gets a hard and safe workout, that challenges their core and embraces their unique body and abilities.