While the knee pinch allows us to off balance our opponents, weaseling your hand in is still necessary to pry their death grip off of your neck. Someone with a good clamp on you will be hard to move otherwise.
Weaseling the hand in does not often mean simply putting your hand in between their arm and your neck. Often you have to creep it in there, little by little, with small wiggling movements and crawling of the fingers. In a lot of ways, it feels similar to rethreading the drawstring on your gi pants when it comes out. You do it little by little, inch by inch. The off-balancing assists this movement and this movement assists the off balancing. It all works together to dislodge your opponent.