The Rule of Three
In BJJ there are so many things that you train your body to do and understand but your mind hasn't ever really thought about them the fundamental principles behind them.
Now, what the hell am I talking about? Here's some context. Say someone has a top mounted position on you, this is not great for you as top mount is the second most dominant position in grappling and possibly the best position in MMA. The three things you need to know to successfully sweep them or escape the position are:
1) Where is their weight/pressure on you? This will show you how you can successfully manipulate your opponent. If they are heavy on their hips and their hips are over yours, a good bridge will have huge influence on where or how they move. If they're heavy with chest to chest pressure, they'll be easier to manipulate or articulate from their head. Both hip and head position determine body position so figuring out which you will have better leverage with or if they're neutral is a huge boon.
2) Where are their limbs currently? Think of the body as having 5 limbs. The 5th is our head. It can be an excellent tool but is equally important to consider our opponent's head position as well as their other 4 limbs as this informs how they can move. You want to limit your opponent's options as much as possible while allowing yourself maximum mobility, keeping good points of contact.
3) Where are your limbs currently? This is crucial. If the only escape you know from mount is an elbow escape but you can't free your arms from under your opponent, you're fucked. The first step in such a scenario is, still keeping the first two points in mind and figuring out how to free your arms to hit the elbow escape effectively. Where your limbs are determine how you can react and move where you want or need to.
You can use these 3 principles from a top or bottom position. In top position instead of where is their weight on you it becomes, where is my weight on them. As the top player, you can use this to either set up a reaction, force a counter and submit. On the bottom to successfully escape, sweep or submit.
Try it out.