I'm a mechanical engineer.

Follow his math on the slide at ~20 minute mark and plot the Y values yourself. You will conclude that there is a good reason his plot goes downward.

EDIT: Shiva is saying that as you get deeper into Republican precincts the percentage of mixed voter ballots should go up for Trump. But mixed ballots are a mix of Republicans, Democrats and Independents. Why should the percentage of them go from ~0-100% voting for Trump just because they live in a more Republican district? They should not. In fact the data shows that at about 20% Republican district mixed voters level off their support for trump at ~35% and it becomes essentially constant. If you subtract the x-value from that constant - as Shiva is doing - then Y will go negative linearly. Shiva doesn't know what the fuck he's talking about.

I'm a mechanical engineer.

Follow his math on the slide at ~20 minute mark and plot the Y values yourself. You will conclude that there is a good reason his plot goes downward.