This a topic that is probably going to bring a lot of comments.
Basically it's is an offset that is put in a line that is going to heat up, i.e. steam line. The line is mechanically pulled during fabrication opposite the direction of expansion. This is normally done during field fabrication. This has to carefully considered by someone well versed in the art, though I understand that newer piping system design software considers this. Better design of piping systems have lessened, but not eliminated, the need for cold spring. It is normally put in steam lines at the expansion loops.
At one time this was very controversial subject on the design front and the pipefitters. The matter was probably resolved 20-25 years ago as lines grew in size and temperatures went up.
