I like giving revision tutorials. Of all the different kinds of interactions with students, revision tutorials have the highest job satisfaction index (subjective benefit to student divided by amount of work required on my part). They come in clutching terrifying lever arch files, bulging with lecture notes, revision notes and ten other kinds of notes, and a look of panic etched on their features. Then they fire a load of random questions at you, which you try to answer in a clear way without recourse to books. It's almost fun, and you get to feel like some kind of biology guru.
The key is not to introduce any idea that they haven't heard of (or should have heard of, but haven't revised), as it makes them go into a blind panic, and flick through a couple of reams of notes looking for the concept you just mentioned. The best part is when you are hopping between ideas and you suddenly see a light bulb come on over their heads as they make a connection that they just hadn't considered before. Sparks jump between the nodes and they see the beauty of the way things link together.