query related to Luhn_Algorithm_solution:

I got the solution for validating credit card number I want to understand the logic behind second if statement where we put if condition of number*2 > 9 will be multiplied by two and subtracted by 9: num_list = [int(char) for char in number]
num_len = len(num_list)
num_list_rev = num_list[::-1]
double = 0
single = 0
for i in range(0,num_len):
        if i%2 != 0:
               if 2*num_list_rev[i]>9:
                        double = double + 2*num_list_rev[i] - 9
               else:
                        double = double + 2 * num_list_rev[i]
        else:
               single = single + num_list_rev[i] if (double+single)%10 == 0:
          print(f'{number} is a valid credit card number.')
else:
          print(f'{number} is not a valid credit card number.')