How is this code working

#1

In line number 35 of the code in this video i.e. CPP - Maximum Sub Array 2 . The line is
currentSum = cumSum[j] - cumSum[i-1]
when i = 0, the cumSum[i-1] means cumSum[-1]. How is that working?

#2

hey Ayush, this is reason we made cumSum[0]=arr[0] before the loop and than start the loop from i=1.