How can I use BodyForce to make a wall-run system?

I've looked into countless different sites in search for a tutorial on how I would make a wall run system. I used EgoMoose's demo, but I don't understand anything.

#What do I need to know before this?

I've heard that many of those types of scripts use something called BodyForce. I have no idea on how I would make that, but it sounds like it forces the Character to move a certain way.

If you can't answer the entire post, then it would be nice if someone explained BodyForce to me!

#What do I want?

To be honest, I really don't want a script. I actually need an explanation telling me what to do, what each property does etc.

What I really want to learn is how I would 'tilt' to camera. For example, when a player is running on a wall, the camera is tilted 45 degrees. I also want to know how to use this BodyForce to make it so that when the player presses A or D it moves sideways relative to their key.

#Conclusion

Thank you for reading up to here, I really appreciate it! I know there's not much advanced scripters out there, but I hope someone answers this question.

To summarize, I want to learn what BodyForce or whatever property manually moves the player when the player presses a key on their keyboard.

Disclaimer: I do NOT want any scripts. I really want an deep, thorough explanation on what each little detail does. I'm sorry if you have to write a long script for this, but I really appreciate your effort.