During jump, the ALU is wired to calculate "-AC". Only the carry out is looked at. The negation result itself is not used and discarded.
What's the use of this? Well, only in case of all-zero bits, "-AC" overflows the ALU, setting its carry-out line. So in this case the carry out acts as a zero indicator (Z). If we also look at bit 7 of AC, we also know if AC is negative or positive. The combination of these two signals makes all condition codes possible:
With this, a single 74153 chip can decode the condition code from the opcode, compare it to the contents of AC, and determine if a jump is needed.