To be honest you got me on that one. I don't know the precise mechanism. There are lots of trials with aloe vera when it's in skin cream or after sun, but the preparations are made in very different ways. There are lots of problems with evaluating the effects because you are not sure you are comparing like with like. The plant originated in North Africa, and has since been exported all over the world because people realised its beneficial effects. It has a substance called anthraquinone, an irritant to your gastrointestinal system that works as a laxative. It also has an aspirin-like agent called salicylic acid, which makes it a good pain-reliever. It also contains magnesium lactate, which blocks the synthesis of histamine. Histamine is one of the classes of substances that make your eyes itchy, that make your chest tight and wheezy and causes a skin wheal when you have an allergy. But because Aloe Vera binds water well, especially if you mix it with propylene glycol in skin cream, it keeps your skin nice and soft and smooth and damp. All these factors (except possibly the laxative effect) explain why it makes an ideal after-sun ointment.