A Hitachi harness is designed for a full-sized Magic Wand like the Plus, Original, or Rechargeable. Most of them will not fit the smaller variations like the Mini or Micro.
More About Hitachi Harness
Hitachi harnesses are popular in BDSM play where a dominant partner might use one on a submissive partner for forced orgasm play. Because of the strength of the Hitachi, it can be overwhelming for many users, attaching it so that it cannot be removed from the genitals can create an extremely intense experience. This type of scene should always be clearly negotiated beforehand.