A Sunis a move where the diabolo swings around in a circle in some way. You can swing a Sun In front of or behind the body or between the arms.
Spin the diabolo then step forward & turn 90° to your dominant side so that you are looking at the diabolo side on. Point both arms out in front of you holding the handsticks so that they are pointing towards each other with the tips level at chest height. Bring your subordinate hand back so that the handstick is pointing at your dominant arm's elbow. Now pull the diabolo towards you so that it swings up between the tip of your subordinate handstick & your chest (watch your face if you have a long string! Hint - keep your dominant handstick at arm's length). When the diabolo reaches shoulder height flick both sticks away from you to pull the diabolo up & over the top of both handsticks.
Throughout the move make sure that the string doesn't wrap around either of the handsticks. As the diabolo swings round the string needs to pass by the tips of each handstick. Turn to face forwards, bring the diabolo back under control & apply more spin if necessary. The string shouldn't be crossed at all, if the string is crossed then the diabolo didn't pass under &/or over both sticks.
Once you're happy with that turn 90° to your subordinate side, point both arms out with the handsticks pointing in. This time bring your dominant hand back so that the dominant handstick is level with your subordinate arm's elbow. This time swing the diabolo away from you. After it passes under & rises to be level with the subordinate handstick, gently tug both sticks back to pull the diabolo back towards you over both handsticks. Let it swing down between the dominant handstick & your chest.
Practise these moves again & again. They are easy to do but it takes time to do them well. The idea is to be able to swing the diabolo by applying the least amount of force & by moving the handsticks as little as possible.
Now work on putting the two together. Start on one side then while the diabolo is swinging downwards turn to the other side. Keep your feet facing forward, lean over the handsticks & only turn your upper body from side to side. Each time you turn both handsticks change from pointing forward to pointing backwards, always make sure that during each loop the diabolo passes under & over both sticks.
This move combines well with other Sun in front & behind the body moves, because the move doesn't cause the string to cross over at all it can be slotted anywhere into a sequence of swinging tricks.
What about the other way?
Yes, you can do a Sun between the arms swinging it in the opposite direction & it is something worth trying. However, this forces the diabolo to go against its natural spin. Imagine a ramp running from your subordinate foot up to your dominant hand. If you place a diabolo at the top of the ramp it will roll down picking up speed as it goes. Now imagine a ramp sloping up from your dominant foot towards your subordinate hand. If you rolled the diabolo towards it along the ground it will get to the foot of the ramp, start to climb, slow down, stop & then roll back again. Swinging in a clockwise direction will add spin to the diabolo, swinging it the other way will cause the diabolo to slow down & stop.