The first thing that you need to do before attempting to do a backscratcher air on a wakeboard is to perfect your wakeboard jumping ability. Unless this particular ability of yours is solid, you will have a really tough time learning and perfecting backscratcher air. Jumping is one of the basic skills that a person is expected and needs to add to his arsenal if he wants the wakeboarding experience to be an exciting one.
Once you have learnt and solidified your wakeboard jumping ability, you can proceed to learning the backscratcher air. The trick is fairly simple though you will need plenty of practice to really get the hang of it.
Start off by moving yourself outside of the wake. In order words, you need to move away from the trail that is left in the water by the moving boat that is aiding you in wakeboarding. This is not exactly part of the trick, but it is imperative for you to do this in order to prepare yourself for the next step.
After you move outside the wake, begin approaching it again with a lot of speed. While learning to move towards the wake will not be something that you would find hard to learn, approaching it with great speed is something that you can only learn through a lot of practice. You will find yourself moving with greater speed as you give it more tries and get the hang of it.
As you hit the wake, pop yourself, along with your wakeboard of course, into a big air. This is where your wakeboard jumping ability really counts. If you have not learnt to jump yet, you will neither pop into the air and even if you somehow manage to do it, the air-time would just not be enough for you to successfully pull-off the backscratcher air.
Once you pop into the air, bend your knees around 90 degrees in order to lift the back of the wakeboard behind you and lean back for balance, which will also complete the back-scratching effect.
As you begin to descend, straighten your legs. However, bend them a little again to absorb the impact as you land on water.