What Does a Rack Feel Like? by Sparks
I have had a fascination with racks since early childhood. I built my first one when I was in my early teens by adding extensions with some pulleys and a crank-winch to a workbench. The toughest job was finding someone to try it on me, but fortunately (?) I had a friend who was as weird as I was. 
All of my racking experiences have stopped well short of the tearing muscle, popping tendon, ripping joint stage, and since I have some long-term plans for the use of my body, I'd just as soon keep it that way! To me, the early stages are actually quite pleasant if proper cuffs are used (good suspension cuffs on the wrists, inversion boots or medical traction straps on the ankles).
Initially, the sensation in the arms and shoulders is a lot like hanging from a pull-up bar, and I feel a sense of relief in my spine as the normal pressure is stretched away. The first place that I feel a dull ache is usually in my shoulder joints, followed by elbows, then knees. The ache in the lower and mid back comes on slowly, sort of a pressure that gives way to a throbbing, then a burning sensation. Placing something under the back to arch it tends to concentrate the sensation more in the mid-upper back (and also introduces an ache in the chest and increased effort breathing).
On a "good" rack, which is pulling from both ends, the muscles of my thighs usually spasm, producing an agonizing pain in my legs and groin (like a charley horse (painful muscle cramp in the leg, foot or arm) that you can't move to relieve). The beginning of shoulder dislocation (you can actually feel the joint beginning to separate) is about as far as I'm willing to go. This is, to me, the ideal "fantasy trip" point, where I can imagine what the next stages would be like. Also, if left at this level of tension for a while, the pain in the limbs gradually eases and the pain in the back and pelvis changes from a dull pain to an intense burning pain.
The options at this point for an imaginative dom are almost unlimited. Sharp pain (cropping, nipple clips, hot wax) contrasts strongly with the dull stretching pain. Electricity applied to the legs or genitals can produce a number of different pleasantly unpleasant effects.
In "actual" use of the rack for interrogation, the victim would often be left overnight at this point to think about his future. By morning, all of his joints feel like they are on fire. The stretched spine is producing intense pain at each of the disks. A few clicks of the winches will completely dislocate the shoulders (if they haven't dislocated themselves as the muscles became exhausted) and possibly the hips as well. For a different, and possibly worse, effect the torturer might release the winches quickly, allowing everything to snap back, causing sudden, intense, agony in the spine and joints.
Of course, to keep from becoming bored the torturer might amuse himself with red hot irons or pincers, thumbscrews (though the hands and feet probably lost all sensation some time ago), or assorted scourges.
If the torturer isn't in a hurry, this process might be repeated over a period of several days. By the end, the victim might measure as much as a foot longer (wrist to ankle) than when the treatment started. Every tendon and muscle in the limbs is torn beyond use. The discs of the spine have separated and the spinal cord is swollen and raw. If damage to the spinal cord doesn't kill the victim (it often won't, because the damage is mostly in the mid-lumbar and thoracic areas), the most likely cause of death will be exhaustion of the muscles required to breath (not unlike crucifixion).