Trouble understanding simple recursion in c

My function:

int num(int x) 
{ 
   if(x>0) 
     num(x-1); 
   else 
     return 0; 
   printf("%d",x); 
}

This function takes a input x and prints the numbers from 1 upto x.I can't seem to understand how this works, can someone please elaborate. Thanks in advance!