Yikes - that one's pretty tough to try to cover in a blog post... My mentor Wayne Gilbert actually wrote up a very interesting article quickly discussing some of the more important aspects of internal and external forces, so I'll point you to his article from one of our newsletters back in March of 2008:
The most important thing for putting "force" into your poses is a careful study of weight, and how the position of weight will affect (and either help or hurt) the action. For example, if a character is pushing against a wall, you wouldn't normally have that character push with their butt either above or behind their feet, because they aren't going to get very much "force" going through their arms. With the butt behind or directly above the feet, the character is pushing merely with the strength of their arms and shoulders, which isn't going to be very much.
However, if they put their hands against a wall or object, and then put their butt (their center of gravity) in FRONT of their feet, well now it's a whole new story! Their weight wants to fall down, right? If you lift up both of your feet, what happens? You'll be on the ground in no time. Well, if your weight is way out in front of your feet, trying desperately to fall to the ground, and the only thing that is holding you up now are your hands against the wall, well guess what?!
Now all of your body weight has become a force that is pushing up through your spine, through your shoulders, and into your arms, and *against* the wall! That's potentially hundreds of pounds of extra force that weren't being pushed against the wall when the center of gravity was above or behind the feet.
This is just the tip of iceberg when it comes to force, but that's a very simple illustration that Wayne used back in the day to help me start to understand that when doing pretty much any action, we choose the most efficient and lazy way to create the most powerful forces, in order to most easily accomplish whatever the task at hand may be.
We use the weight of our bodies to get the best "bang for the buck." We use our weight to start walking, run faster, punch harder, throw further, and lift higher. Any study of internal forces has to begin with learning how and where we choose to place our weight in relation to whatever the action at hand may be.
Hope that help someone! Thanks again for coming by!