His effort during Manchester United's 4-0 win at the Stadium of Light led to a number of suggestions being put forward about how Ronaldo manages to get such swerve and dip on the ball.
One theory is that he tries to make contact with the valve on the ball, which then moves around violently making it almost impossible for a goalkeeper to stop.
But Ferguson dismissed the notion as "rubbish", insisting Ronaldo benefits only from the extra work he does on United's Carrington training pitch.
"All I have heard about over the last day or two is about the ball and how Cristiano shapes up," said Ferguson.
"It is rubbish. He takes about 30 free-kicks after training every day. Not only are the balls not the same ones we use in matches, I have never once seen him pay any attention to where the valve is.
"The most important thing is practice. Practice makes perfect."