A lot of us vote on machine, but it also prints a paper copy that is submitted as well. Because the copy is produced by machine, it can be encoded to be directly and accurately tabulated by machine initially, but also can be hand counted, if need be, later. Speaking as a software engineer, exclusively electronic voting is a fucking terrible idea for any number of reasons, like malice, incompetence, good old human error, etc. But we don't do that. There is and should always be a paper trail to validate it.