Rather than relying on the traditional tensioned and sprung flat steel plates of the spring axle, torsion trailer axles take a different approach entirely.
No metal springs of any type are used; rather, a series of long rubber cords are situated inside a square tube with an inner core piece that the torsion arms are secured to. This type of axle can be difficult to understand just by reading about them. In this case, a (moving) picture really is worth a thousand words:
As you can see, the above video shows the cross section of a torsion axle. The outer square steel tubing is the part that's actually secured to the frame. The inner square piece is secured to the respective wheels via the torsion bars.
When the wheel moves up and down on bumpy or uneven ground, the inner bar compresses the rubber cords along the length of their bearing surface. The contact points are simply the corners and edges of the center support. Unbelievably simple!
Benefits of the torsion axle include:
Maintenance free design. Lube your wheel bearings and that's it!
No metal-on-metal contact points
Independent wheel suspension
Quieter during travel
Cross-bar construction is directly welded to the frame, and adds stiffness
Warranty typically lasts longer than leaf spring axles