This has to do with the design of the zoom lens. On some zoom lens, the lens barrel actually extends further and further during the 'zooming', i.e. the overall lens become longer. For the better zoom lens, the designers have managed to achieve the zooming effect while keeping the length of the lens constant (meaning lens barrel does not extend) during the zooming. This is prefered and makes handling more pleasant. For closeup work, it keeps the lens to subject distance unchanged throughout the zoom range. Most of the better and pro zoom lens would have this feature, e.g. the Nikkor AF-S VR 70-200mm/f2.8G, the physical outer dimensions of the lens remain unchanged throughout the zoom range. Hope this helps.
i'm using tamron 17-50, when focusing, the length of the lens barrel does not change. Unlike my previous kit lens or 50mm prime lens, when focusing, then len barrel extend in & out.
So i suppose the features you mentioned is the same concept, but zooming(also including focusing) instead of focusing?
meaning the len barrel won't be extended when u do zooming or focusing, everything is internaly within the len barrel.