Game changer...I am a very active below knee amputee (work full-time, play golf, workout, etc.) and for years, I was told not to wear anything underneath the liner for my prosthesis as it would interfere with the vacuum system that helps secure the socket to the residual limb...boy, was that some bad advice...the swiftwick Valor not only aids in increasing vacuum but the sock also helps with moisture management (I live in Arizona where heat/perspiration is an issue)...by increasing the vacuum, I have a more secure fit with less issues with sores on my residual limb due to the Valor sock assisting with moisture management...big fan of the Valor socks and will be sharing my experience with my prosthetists so they can share with other clients.