Everyone has seen this trick. A diaboloist steps over the string & circles the diabolo around their thigh. The trick is really no different to the basic Circular throws except that you insert a leg.
As usual I like to start each tutorial with a bit of stating the obvious. In this trick the diabolo is thrown up from one side of your leg so that it flies over the top of your thigh, it is then caught on the other side of your leg then it rolls down the string underneath your thigh to the other side ready for the throw again. As explained in the circular throws tutorial throwing the diabolo one way will speed it up while the other direction will slow it down. To build up spin throw the diabolo round in a circle in the opposite direction to the way that the diabolo is spinning. So if the diabolo is spinning anticlockwise then you throw it in a clockwise direction.
Secondly you need to decide whether you want to do this trick standing on one leg (if so read this Advanced guide to standing up for some mostly useful tips on balance) or with both feet on the ground. It is possible to throw the diabolo around the leg with both feet flat on the floor, this method is how you will have to do it if you want to learn to throw the diabolo around both legs later on. To do this trick with both feet on the floor then stand with your dominant leg stretched out straight in front of you & with your subordinate leg a step behind it. Lean back so that most of your weight is on your back leg.
Begin spinning the diabolo, then lower your subordinate hand so that the diabolo is spinning closer to the subordinate rather than your dominant handstick. Step over the string with your dominant leg so that the diabolo is still spinning underneath it on the subordinate side.
Pull up with your subordinate hand so that the string forms a 'V' shape with the handsticks at the top & the string looped under your leg at the bottom. This movement will throw the diabolo up & across to your dominant side. This only needs to be a little throw so you don't need to really heave at it. While the diabolo is in the air lower your subordinate hand & raise your dominant handstick up to meet the diabolo being sure to keep the string held tight between the handstick & your leg as you do so. If the string is loose it will flap around & will be more difficult to place on the axle. Catch the diabolo on the string then lower your dominant hand a little to allow the string to slacken off so that it is not being held up against your leg, this will allow the diabolo to roll down the string & under the leg ready for another throw on the other side.
Start off with just doing one throw & catch at a time, then with practise work on using the momentum of the diabolo as it rolls down the string under the leg to go straight into the next throw.