The Express has a specially-designed narrowing of the webbing at both ends. This ensures that the karabiner is correctly loaded which may not be the case with wider webbing. Comes complete with a Petzl String to hold the lower krab firmly. Being polyester, they are harder wearing and should last longer than Dyneema extenders but they are a little heavier.
They are also easy to grab and hang on to if you have to!
Probably the best climbing wall and sport route extenders.