Ronaldo hits his freekicks with the top of his foot which gives it a lot of power and swerve. Its a very hard method to use because you can easily get too much height on the ball. Beckham used to hit it with the inside of the foot which gives it a good curl but Ronaldo's swerve and move in midair, they are very hard to save. The back steps however, seem to just be and approximate way of measuring the appropriate distance for the run up. The rest is just personal preperation. Only through days and weeks of practice can you understand the technique behind it.

"My free-kick secret? I just look at the net and say 'take the kick, Cristiano'."

straight outta Ronaldo's mouth, his secret to free-kicks.