The CPU Cooler is a features that decrease your device temperature identifying apps which are heating the CPU and shutting it down.
To use CPU Cooler on your phone, you have two options:
1) Open DFNDR Performance, tap on CPU Cooler shortcut located at the bottom of the main screen.
2) Open DFNDR Performance, tap on TOOLS tab and select the CPU Cooler feature.
3) Wait until your device cools down. Both paths will execute the same process.