In the 1990's the term X-Factor was coined by instructor Jim McLean. He even wrote a book that talked about this idea and it was a popular read among avid golfers. The meaning of x-factor is simply the difference between the amount of hip turn vs shoulder turn at the top of the back swing.
Studying and teaching this idea has some merit among tour players, but for the average golfer it is been a messy subject and one that has caused much confusion and sore muscles. Tour players simply create a large amount of x-factor because they are naturally more flexible than average golfers. They can easily turn their shoulders much farther than their hips and it is an effortless endeavor. The average player is not very flexible, and trying to create this stretching or torquing in the back swing is almost impossible.
For an average player, the x-factor is created naturally when they change direction from the back swing to the down swing. This transition creates the torque and stretch simply because the hips and legs start moving back toward target before the upper body gets moving.
If you think that you should hold your hips still on the back swing and crank your shoulders against your quiet hips, be very careful. This is your attempt at artificially creating x-factor and it will eventually cause you to be very sore and tired. Instead, allow your hips to move freely in the back swing and your will find your swing will work well and your body will feel great.