WATCH INSTRUCTIONAL VIDEO BEFORE USE. Customizable level of support. FAST stands for Functional Anatomical Stabilizing Technology. This methodology is inspired by the body to move the way athletes move and protect the way the body protects. The primary three parts of the IFAST are designed to mimic a part of the body. Total Performance with Complete Ankle Protection. Moves the way athletes move and protects the way the body protects. Low profile, in the shoe, dynamic ankle stabilizer. We recommend using the IFAST on both ankles to help reduce the risk of injury. Medium: Mens 9.5 - 11 | Womens 11 - 12.5.