How? Because the voting machines all report their votes to a centralized tabulating computer. A man-in-the-middle attack would insert a computer between the voting machines and the tabulating computer. The voting machine still thinks it’s talking directly to the tabulating computer, which still thinks it’s hearing directly from the voting machines, but in reality, the voting machine talks to the man-in-the-middle computer, which then reports whatever the heck it was told to report to the tabulating computer. Neither the voting machines nor the tabulating computer have to be directly hacked for this to occur.