When we’re playing guard in BJJ we have few simple rules like you need to have 4 points of contact with your opponent, have your knees opened etc… But one of the most important one is to not let your opponent control both your legs at any point. If he’s controlling one it’s survivable because you can always defend with your other leg. But if he controls both he can just manipulate with your hips too easy.
And in MMA it’s even worse because once your oppponent pass your guard you’re literally screwed. Here is an example.