You are correct, no one knows "exactly" how the elections are classified as "safe"....there are a lot of theories, but they are only theories until things are made public. Trump set up a commission to preserve the integrity of elections ... https://www.npr.org/2018/01/03/575524512/trump-dissolves-controversial-election-commission , but because all the Governors of Democrat states refused to supply information, it was disbanded, but the job was turned over to the DHS .. https://www.usnews.com/news/politics/articles/2018-01-05/trump-ends-election-integrity-commission-but-voter-fraud-probe-continues ....They have had a lot of time to collect data, so perhaps it's all about to be made public