Intersting thing though... WhiteTrash, MrDinkleman, and YOguyDA, are right. The only point that I take issue with in aaron_sk's response is that a torsion CAN be progressive. By tapering the bar, you can allow the first part of the bar to twist easier than the next section which will twist easer than the next section and so on. I've only heard of it in long travel torsion bar setups for desert racing, but never bothered to check either the stock bars or the Sway-a-ways that I ran for a few years.
Everyone has their own opinion. And i understand everyones ideas, but anyone can go push down on the front end on a stock height truck, lower it an inch and push down on it a few times again and it WILL be softer. Why? Because the bar is relaxed a bit(some preload has been taken off the bar) It is what it is.
That isn't preload. That's the angle of the lower arm changing. When the truck is at stock height the lower arms are pointing slightly down from the torsion bar. When you push down on the truck the surface of the tire tries to push outward against the road until the angle of the lower arm is level. Once the arm is level and above there is a transition to the tire surface from skidding outwardly against the road to neutral then inwardly due arch of suspension travel. Remember the tires don't move straight up and down, they gain negative camber as the suspension compresses.