Sometimes you are in a position and in your mind you can see yourself doing something but when you try it, it just doesn't work. It's because of friction. The friction of their body against yours. The friction of your gi against theirs. How someone creates and controls the friction becomes a huge advantage. Both people put the same amount of weight on you but that invisible part is the friction one person creates.
The Inner Game of Jiu Jitsu - Friction
My Journey Through the Art of BJJ: This is not an Instructional