Rotary epilators are commonly called rotary tweezers. They look similar to electric razors, but instead of a sharpened blade on the rotary head, they have at least two rows of tweezers; which rapidly grasp hairs and pull them out from the root. But before you run screaming for ice - this process isn't as painful as it sounds.
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When a rotary epilator works, the rows of piranha-like tweezers spin quickly enough to pluck out all of the contrary hair along the way. So it's hardly more painful then a typical tweezing session, and its takes half the time. However, like tweezing, rotary epilators don't destroy the hair follicles, so they only remove your unwanted hair temporarily. The hair will re-grow in a few days and up to a few weeks.
Rotary epilators work best on unwanted arm and leg hair, where the skin isn't too delicate. For fine hair, it's difficult for rotary epilators to grab at the hair. Also, to use a rotary epilator effectively your unwanted hair has to grow to a certain length for the tweezers to grab it and remove it.
Rotary epilators are not recommended on large areas such as chest and back. In addition, plucked hair can cause irritated skin and ingrown hairs, so after your epilator session wash and moisturize the area.