Having your arm bent is also the strongest position in terms of grip and endurance too (as is taught in Tai Chi and martial arts!) so this is a good way to ensure you don’t accidentally let go of the rope and let yourself fall over. By keeping your legs slightly bent you also give yourself the ability to extend them or contract them as you need to to react to the waves as you go over them.
To balance you should try and keep your centre of gravity fairly low to start with. Bending at the hip can help you to achieve this, as can slightly leaning forwards. It’s possible to practice how to wakeboard by using a balance board or another similar tool in order to learn how to balance on a moving platform.
Wakeboarding is called wakeboarding because it involves both a wake and a board. For those not in the know, a wake is the name of the small wave caused by the boat as it cuts through the water.