Here is the hard part, you have to blow all of the air out of your lungs and bend forward until you are completely submerged. Make sure to get all of the air out because any extra air makes you appear fatter. You wait about 5 seconds under water while your weight is taken. Your underwater weight is figured out by a mathematical algorithm that determines your bodyfat. This test is based upon the premise that muscle sinks and fat floats.
Muscle is denser than fat therefore it sinks.
The margin of error on this test is 2 percent.