The gist of it goes like this:
1. In the last several elections, it has become common for democratic candidates to gain votes as absentee/mail-in ballots are counted, which often is up to a few days after election day. Generally this does not change the outcome, but in some recent elections the republican candidate is ahead (by a bit) on election day, but this changed as absentee ballots are counted. The situation is likely to be more extreme this election as (1) most democratic voters (60% by one recent poll) plan to vote by mail due to the pandemic. That means more ballots to count, which will delay the final tally. On the other hand most republicans plan to vote in person (only 28% mail-in by the same poll), so republican candidates are likely to be leading on Nov 3.
2. The Trump campaign (assisted by Barr) has been trying to discredit mail-in voting. The idea is both to get as many ballots disqualified due to trivial errors such as not having a second "privacy envelope", on the theory that this will disqualify more democrats than republican voters. However the basic idea is to create confusion and then use that confusion to try to get all mail-in ballots thrown out.
3. Also they plan to go to court to get counting stopped on Nov. 3. Even if you filled in your ballot correctly and mailed it in lots of time, it will be disqualified if it is not opened and counted on Nov 3. This is very likely to affect lots of votes because of the increased volume of mail-in ballots, the lack of resources to hire enough people to count them all quickly, and in some states there are laws to prevent mail-in ballots to be counted before Nov. 3. Even if they are received weeks before Nov 3 they will just sit unopened.
4. If all of the above does not produce a vote for Trump, they will go to court to argue the votes are so "tainted" they cannot be trusted, so they should all be thrown out (in-person and mail-in) and state legislatures should choose electors for the electoral college. Since slightly more states have republican legislatures, this should ensure a Trump victory.
5. There are a few states with republican controlled legislatures but democratic governors. In such a case the legislature can choose a set of electors, but so can the governor. However no state can send two sets of electors. In the event of a dispute, who gets to choose what set of electors to count? Why, the president of the Senate, who happens to be Mike Pence. Any guesses which set of electors is counted?
The article goes on to discuss even more scenarios, all terrifying for the future of our country and all quite probable with this president. Things like using protests to send in troops in democratic-leaning cities to close the streets so voters can't get to the polls. Or worse things like inviting "citizen militias" to police polling stations and keep out "suspicious looking troublemakers". And on and on.
I had planned on voting by mail, but now I am thinking about voting in person, but early to avoid some of the crowded lines.