The freewheel on the MX Rev is a mechanised flywheel concept.
By clicking it or setting specific apps in its software a little motor "releases" the wheel so it is entirely free wheeling, seeing as its a weighted metal wheel, it works like a flywheel, keeping rolling for aggggggges if you give it a good flick.
Alternatively if you dont wish it (in games etc) then it becomes a normal "click by click" scroll wheel.
Its all very clever and once your used to it and have all your apps set up in the software as you want them, sooo useful. Its perfect as has been said for quickly scrolling in 3d apps that use the mousewheel for zoom as you only need give it a small flick and away it goes, yet in PDF's and Word documents etc the normal click by click works far better so you set it in the SW to change to that when those apps are loaded.
Its odd feeling the little click in your mouse as the mechanism re-engages to give the feel of click by click as you open Word etc but you soon get used to it.