Knee on stomach is one of the most painful and fustrating positions to be in. On the top you can control your opponents hips and shoulders. If your opponent tries to escape you can pull up with your hands and force your knee to the ground.
Numerous chokes and arm-locks can be applied from this position.
Although called 'knee on chest', you may have your knee on your opponents stomach. With your right hand holding their belt and your left hand holding their collar using your left foot for a solid base. In this position you are simply sitting on your opponents belt, to apply force simply come up onto your knee slightly.
A close-up of the hand and knee positions.
You may hold either side of your opponents collar.
In this variation you control your opponents right arm. Your right hand holding the collar (fingers inside, thumb outside) and hold their elbow with your left hand. Remember to use your left foot for a solid base.
A close-up of the controlling hands position.