It is important to point out at the outset that, although often practised by students around exam time, it is not recommended as a serious revision method, there being no proof that it actually works. If you want to pass an exam, there is no substitute for hard study.
'Revision osmosis' mostly consists of spending copious amounts of time in your room, to give the impression of revision, while actually surfing the internet (or, basically, doing anything other than revision). But don't worry, it's okay, as long as you're in the same room as your textbooks and notes you're almost revising, which must be nearly as good as the real thing, right1?
There is a school of thought which believes that 'active revision osmosis' may be more effective. This involves trying to 'push' the knowledge into your brain by hitting yourself on the head with various textbooks. However, caution must be exercised when using this method, as concussion is never a good thing when trying to pass exams. Another form of 'revision osmosis' is to write all the important facts you need to learn in a big font on sheets of paper and pin them up around your room, the reasoning being that you read and learn them without realising it.
There are, of course, other 'alternative' types of revision, such as the much-used 'staring at one page of your notes for hours on end while daydreaming about something else' method. There is also the 'flicking back and forth through your notes so fast you can't actually read anything on the page' method. Both of these are sub-divisions of the 'revision osmosis' method, however, true revision osmosis has the advantage that you can be doing something more important2 at the same time.