The Zipser Clamp (sometimes referred to incorrectly as a "Zipster") is a simple spring-loaded tool designed to reduce bloodflow to the penis. By doing so, it makes performing procedures such as a meatotomy or subincision much easier. It would not normally be used on piercing procedures (since these don't tend to get bloody, and there's no real need for this tool), although it is sometimes used in the case of larger gauge or scalpelled genital piercings.
Normal use is as follows: The penis is exsanguinated, and then the clamp is placed over the shaft of the penis above the location of the operation. The clamp is then removed after the procedure, allowing normal bloodflow to resume.
If an anesthetic is not being used, the Zipser will also reduce pain, to some extent.