There are many important factors in generating power in the swing.  One of the most important is the loading of the hands.   More specifically, the timing of this step in the swing is vital to achieving maximum batspeed.

The video below shows JD Drew going through 2 distinct phases of the swing.  The first is the load.  During this phase the hitter is shifting weight onto their back leg to prepare their forward weight shift.  The second phase that is shown in this clip is the stride phase.  This is when the batter shifts weight to the front side to get into the launch position.

Take a look at the clip below.  Watch the hands during both phases.

Notice how when the weight shifts back, the hands stay quiet.  However, when his body begins the stride phase, his hands move back.  This is helping him create hip/hand separation.

More clips below. Watch how when the body starts to move forward, the hands move further back and the distance between the front hip and the hands increases.

Why is the timing so important? 

It has to do with the stretch shortening cycle or SSC.  Basically, the SSC is when the body stretches a muscle right before it fires it.  This process releases power. Think about a rubber band.  The more you pull it back, the more power it will create.  Essentially, this is what is happening when we move the hands back as the body moves forward.  

What is stretching? 

Fascia.  Fascia is connective tissue that surrounds the muscles and links from our head to our toes.  There is a band of fascia called the front functional line that connects the right shoulder with the left hip and vice versa.  When a hitter load the hands as the hips move forward and begin to open, it creates a tremendous amount of stretch in this band of fascia and helps create power.

However, the timing is vital.  The shorter the time between the stretch and the release, the more power is released. This is why a hitter should move the hands back as they stride.  This means that the distance between the front hip and the hands should be at their greatest distance right before rotation starts.  If this occurs too soon, a hitter cannot take advantage of the stretch shortening cycle and will be limiting batspeed, and in turn, power.

How to work on this

You can work on this either with or without actually swinging at a ball. Simply get in your stance and work on the load phase and the stride phase.  Make sure that as the body shifts back, the hands stay quiet.  Then, when striding forward, the hands move back away from the body. You should feel a slight stretch in your core if this is achieved.