I've never used one. But evidently they're also known as "juicers" and seem to be very popular in glass. I read a bunch of reviews on this one
. The term "juicer" refers to the fact that most women get extremely wet when one is used on them. It's supposed to be inserted and then the knob on the handle (offset for this purpose) used to rotate the dildo. It seems popular, from what I've read, both for the enjoyment of itself, but also as as a warm-up or foreplay to get ready for larger toys or PIV sex. Glass ones I've seen, like the one I linked, have the swirls on them as well as divits out of the head that create a unique sensation when rotated. They also have raised nubs on the round base that are supposed to be able to stimulate the clit as it turns, but this seems to be unique to each woman's specific anatomy as to whether it works or how effective it is. Some women rave about the nubs while others don't feel a thing. The're short, the idea being that the head will rotate against the g-spot, though whether it will depends on the anatomy of a particular woman, of course. The other benefit, from what I've read, is that these are hard material, glass or metal, which means that normally, you would have to be quite cautious with thrusting them, but because they are short, they make good thrusters for people who like firm materials and thrusting and don't want to worry about holding back to avoid bruising the cervix.