In some piercings, especially those in areas of high movement, flexible jewelry may be used to reduce the risk of rejection.
An alternative method of doing surface piercings is to use some form of flexible bar. The theory is that because the jewelry flexes and moves freely with the body, it will cause less irritation than a solid bar. While this is partially accurate, there will still be more pressure on the exit holes than with true surface bars, and the piercing is still likely to migrate or reject.
Flexible jewelry is also sometimes used in industrial piercings to reduce the pressure on the piercing holes, although a custom bent metal bar will have the same effect. Pregnant women sometimes choose to switch their navel jewellery to something flexible to reduce discomfort as they start to get larger, but this still might not be sufficient to keep the piercing healthy, so many women simply prefer to remove the jewellery and reinsert it after giving birth.
Note also that not all flexible jewelry is created equal. The two most common materials are probably PTFE and Tygon (a kind of plastic tubing). Of the two, Tygon is drastically more flexible, but needs to be changed every few months. Nylon (fishing line) is also sometimes used, though this is becoming increasingly rare. Of the three, PTFE is the only material approved for long term use in the body.