Arduino Uno code explanation

Hello,

I just build the shield and tried the code source on my arduino and everything seems to works fine

I just wanted some explanation of the arduino code, especially the ISR part.
Indeed, i guessed that interrupt only occur when there is a trigger on the Pin ICP1, or when the ACSR registrer is correctly configured to be triggered via the analog comparator, but there is nothing wired to the ICP1 and the ACSR is not configured, so ... how this ISR is triggered ?

Maybe I missed something in the registers configuration ?

Thanks for your help in understanding this