Recall that the way addition is performed at the bit level discarded the carry out of the leftmost bit position.

Mathematically this changes the value of the addition.

Mathematically if we keep this carry out of the left most bit, instead of getting 0 when the two's complement is added, we get the value 2ⁿ where n is the number of bits in the integer type.

Why? Because the bit value of bit position k integer is 2^k. Since the bit positions are number from 0, the leftmost position of an n bit integer is position n - 1.

The carry position would then be position n.