These targets are extremely painful as well as dangerous. Hohan Soken labels them as Fatal, due to rupturing the liver or stomach, depending on which side you strike.
Striking with lighter force so as not to rupture, will induce severe internal pain that can inhibit motor function, nausea and even altered levels of Consciousness. Being the recipient f this with a well placed strike will instantly drop you to the floor, with incapacitating pain. This can last for a number of weeks depending on the severity of the blow.
To learn the method, targets and see the affect in real time, the extended films are located at these links: