First of all there's no such thing as just a yo-yo - its either high or low. Secondly a yoyo is a RELATIVE move; it describes a move which is relative manuevering to /around a bandit - eg it doesnt exist outside that context.
Both yoyos are moves to try turn with a bogey who can turn more effectively than you in a single plane (eg a flat turn, for example).
A high yoyo is a move you'd employ when your too fast to turn 'in plane' with the bogey. Say you dive on a zeke and attack from the rear, and he break turns. Now because your much faster than he is you wont make the turn, so you do a climbing turn to slow down then roll and pull down on the bogey's 6 again. Thats pretty much a high yoyo. It looks similar to a chandelle. Your converting your excess speed back into alt (conserving E), and now you have lost excess speed you can turn a lot tighter, then you roll and dive and regain your original speed (or 90% of it). It doesnt work to well for bogeys who do extended turns, but you could try repeated yoyos in that case?
http://www.flightsimbooks.com/jfs/page100.phpA low yoyo is a move you'd employ if your too slow to turn with or keep up with a bogey if you were to turn 'in plane'. This is the opposite of the above. Say you near stalling and cant control your a/c enough to continue to turn with the bogey. In this case, you nose low and do a dive-then-climb scooping turn in pure or lead-pursuit. The dive gains you the speed you need to turn and then you can hopefully 'cut the corner' and effectively turn with and sometimes catch the bogey and blast em.
http://www.flightsimbooks.com/jfs/page98.phpIn the provided links ignore the missiles bit...
