In Sphinx, the pose is held very passively. We allow the shape of the pose to do the work. However, in Seal, we can increase the sense of stress (via compression) in the lumbar spine by changing the position of the hands. As the hands come further back (to more under the shoulders) there will be more curve in the lower back, thus more stress on the vertebrae.
Another option is to push into the hands. This works best if the hands are not quite right under the shoulders. If the hands are a little forward, we can push the hands into the floors, which will increase the stress in the lower back. In this respect, yes we can increase the sensation (which I think you mean when you say traction, although traction is usually a pulling apart rather than a pushing together.)
It is not necessary to push into the hands in Seal, however it is an option. Do what feels juicy, not painful. Remember, time is more important than intensity: you do not have to be a the most intense level of stress, but rather hold for as long as you can.