Kinda curious if that is just a ThinkOrSwim thing they push for their retail traders to do. Seems like it would generate a ton of commissions. Never really looked into it...

Is it something any professionals or prop traders do? Seems like it would be much harder than spreads in futures as a lot of stocks are hard to borrow.

yeah, Long short strategies are pretty profitable if you trade one stock long and the other short in the same sector because you can profit regardless of what the market is doing as a whole. It's good for volatile or unpredictable days but It requires a lot of capital because you're trading two positions at once and thus should position them accordingly with proper risk. It's a great technique.

Consider that price movement in a stock consists of three components: 1) general market, 2) sector, and 3) company-specific factors. So when you pair trade two stocks in the same sector (as mentioned in the above post), you are actually cancelling out the first two components, general market and sector, and isolating the third component. Thinking of it this way focuses your analysis on the company-specific differences between the two stocks without regard for the "big picture" influences from the larger market.