Goatears is correct when it comes to the most basic version of the effect though the subjects needn't be seated. As I said, there are many variants to how this effect is created, one of the more clever is Colin McCleods which has not been published. Too, Ted Karmilovich has a very cool handling that's mind blowing.
Once you know the basics (and there are two primary methods) you can let your imagination play with it. . . there are routines, for an example, in which a Voodoo Doll is used vs. the 2nd participant . . . your main helper feels where you put pins into the doll.
Yes, there are at least two gimmicked methods some people incorporate in this routine. They are generally expensive, one far more expensive than the other. Both give "real time" reactions to the touch.