I've never delved into the mechanics of a rack but having done it in RL (under VERY controlled circumstances) I can this about me.
Small incremental stretches are very erotic. You fight against the restraints, you feel you body being stretched and stretched to the point where you can't move. Helplessness sets in. you can move your fingers and toes but that is it. We used leather lined cuffs but I imagine if rope was used you'd feel it cutting into you ankles and wrists. And then there would be stinging in the extremities. The next slight pull makes you feel like you are being pulled apart all over your body....................................
Now we did do one sudden long pull to stretch me completely out after most of the tension had been taken in. That was a sudden intense sharp pain like dislocating a joint (and yes I have. I also tore my ACL playing basketball). Both the suddenness and the intensity of that was quite painful but set my mind into the torture scene. I was able to transition (mentally) from consensual play to torture.
So I think you have to design the machine to do both for the full effect.
kisses
willowfall