This item appeared as a long tube attached to the groinal socket of the mechanoid. It could have a vacuum cleaner, egg whisk, hedge trimmer, buzzsaw or power drill attached to it (and, it is implied, other attachments).
The egg whisk was used to stir Lister and The Cat's tea aboard Starbug when Kryten's behaviour protocols had been deactivated. He did not realise just how inappropriate this appeared. (RD: Tikka To Ride)
Lister once wondered if the length of the attachment was a reference to the endowment of John Warburton, on whom the 4000 Series was based. (RD: Beyond a Joke)
In the "making of" video of "Polymorph" contained on the second disc of the Series IIIDVD, the vacuum attachment is described as being more rude if it had been placed at any other point on the body.
In Series VIII, Kryten is classified as a woman due to a lack of genitalia, distressing him to the point of building an artificial penis called Archie. Evidently, the staff of The Tank never observed Kryten vacuuming...