Flutterby™! :

Sex Toy OTD: The Amazing Chi Machine. You lay on your back and put your ankles on the notches on the machine. It travels about 5 inches back and forth, about two and a half cycles per second. Nominally a device for better health, the brochure has all sorts of gobbledygook about how fish don't get sick and this is because the waving motion of their swimming and all that, but the practical upshot is that this sets up a wave through the body with nodal points at the shoulders and midway between the bellybutton and groin. With a friend... er... helping to stimulate the other senses, tensing all those muscles that normally precede orgasm becomes pretty impossible, and combined with the regular motion things get... well... intense. Not something I'd wanna do all the time, but...