Favoring keys is another way of saying Slow in and Slow out. Or as Mike Disa, my 1st year animation teacher at Calarts would say, Acceleration and Deceleration. Everything in motion must either be accelerating or decerlating in space while in motion, unless the object is completely still. So what you break down the elements in the human body, part by part, you'll start to see that everything is doing this. Once you have this principle in mind, you'll be able to consciously apply it in your animation. It will be the concept that informs how you handle your SPACING. So just remember, everything is either accelerating out of a pose or decelerating into one.
Timing charts are all essentially a blueprint for your spacing. Some animators use a separate timing chart for almost every part of the body so what they're doing is looking at the spacing of each body part as it moves in space, and dictating whether it's accelerating/decelerating or slowing out/slowing in to the next KEY.
You will hear a lot of CG animators these days talk about "tracking arcs". One reason why someone would want to track their arcs is to create appealing smooth/fluid movement but another component of that would be to track SPACING. To make sure everything is slowing out/in or accelerating/decelerating.
For example, when someone gets out of a chair, they must first bend at the waist in order to transfer the weight of their body onto their feet. When the torso first starts to move, you will notice that the frames towards the beginning of the movement will "favor" the first pose. Then the spacing of the torso, as it travels away from the seated pose, starts to accelerate out...creating bigger gaps in the spacing. When the torso arrives at the next key, it begins to decelerate again just as the weight is shifting towards the feet. And as this is happening, the hips are beginning to slow out/accelerate out of the seated position but they will also eventually slow in/decelerate into the next key of let's say, the character standing up. You can break down the spacing of every body part in this way. As everything travels through space, it is favoring the KEY Poses! Hope that helps!