The Flash's alter ego, Barry Allen, is a research chemist who is accustomed to fabricating his own test equipment. The suit-concealment ring and repeatably-compressible suit was a great challenge. The ring is a combination of miniature machining and traditional lost-wax jewelry casting and is made of a high grade stainless steel. For the Flash's costume, Allen developed a thermally stable costume, suitable for high speed travel. The suit is made of highly compressible synthetic silk with ultrasonic welded components. The key to the suit's effectiveness is bonded outer layer of ultra-smooth Teflon which aids in thermal dissipation and eases the high speed passage through atmosphere or other media. The suit fits into the ring through a series of carefully calculated foldings. The suit is compressed so rapidly that great heat is developed as it is placed inside the ring. As the suit cools, a partial vacuum is formed which holds the ring halves closed. The suit forms its own seal against the only opening, which is the release stud. When the ring is opened, the synthetic silk molecules uncoil and absorb atmospheric gases, expanding to full size very rapidly.
Barry Allen gave Flash Costume Rings to other members of the Justice League, containing their own costumes, as seen when Green Arrow has Arsenal steal the one Green Arrow had gotten from Barry, which had been in the Flash Muesum's collection "Friends of the Flash". His costume is stored inside of it and it says "To Ollie, Run fast. - Barry". Ollie makes it into his engagement ring for Dinah. The other members of the Justice League didn't use them because they weren't fast enough to get into their costumes.
In the Justice League animated series, Flash has a drawer full of Costume Rings.
In Justice League: Doom, Barry Allen uses a Costume Ring to change into the Flash.