There are two types of pipe bending spring: external and internal.
Internal pipe bending springs
Internal pipe bending springs are fed into the inside of a copper pipe and help to support the walls of the pipe during manual bending.Using a spring prevents the pipe from creasing or becoming distorted. The ring on the end of the internal spring is used to attach a wire or string so that the spring may be retrieved if it passes out-of-reach inside the pipe.
External pipe bending springs
External pipe bending springs are fed over the top of copper piping and help to support it when manual strain is applied. External pipe bending springs are typically used on pipes with a diameter less than 15mm.
Hook and ring bending springs
Some models of pipe bending spring have a hook and a ring on their respective ends. This allows you to link several together for larger jobs.