On circle track rims there are often two valve stem holes, one for the schrader valve to fill the tire, and one that holds an adjustable pressure relief valve for when the tires get hot and try to expand.
The relief valve leaks out the excess air pressure to keep the desired pressure level.
It is more common now though to use nitrogen instead of air in these race tires, because nitrogen stays cool and does not expand near as much as air does.