Your kidneys have no exoskeleton protecting them, they are not connected to any strong structure but nestled inside fatty tissue which means when your body is being bounced around a motocross track, so are your kidneys.
Prolonged exposure to these kinds of forces will result in pain and tenderness and even blood in your urine. During a crash they are susceptible to impacts from handlebars or other objects, even making contact with another rider during a race can result in a damaging blow to your organs.
A direct blow to the kidneys can cause them to rupture, a serious rupture will result in rapid internal bleeding as the blood supply to a kidney is artery driven so abdominal pain after a crash should be taken seriously and monitored closely by medical professionals.
How does a motocross kidney belt protect me?
The two key functions of a kidney belt are to help keep your kidneys and other organs protected from impacts and reduce the amount of movement your kidneys encounter when riding whilst stabilising the lower part of your back.