An effective decompression force should beable to separate spinal segments to at least 3mm.
The following get involved in traction : Vertebrae, disc, muscles & tendons, ligaments & capsules and nerve roots.

Spinal traction is a form of decompression therapy that relieves pressure on the spine.
It can be performed manually or mechanically.
Spinal traction is used to treat herniated discs, sciatica, degenerative disc disease, pinched nerves, and many other back conditions.
However, Spinal Decompression therapy is different and far more advanced than traction.

※ Above : Early Greek traction device in the 2nd century A.D.