Tech Punishing is a mid level technique that helps you control your opponent's actions (to a degree) by causing them to fear ground teching. It prevents people from mindlessly teching out of the corner, allowing your rushdown patterns to run much longer. Once you've taught your opponent that teching your common setups is bad, you can go into new wakeup setups/patterns that are only an option on opponents who do not tech. You can also try to tempt them into a tech using techiques such as double OTGs while waiting for the tech to inflict much more damage than the double otg would have netted.
What is tech punishing?
Whenever you ground tech, there is a very small recovery window upon completing your tech and standing on your feet. During this window you cannot block/attack/perform any action. This can be exploited by landing any sort of meaty attack so that it hits them immediately upon completion of their tech for free damage.
Even if you 'miss' the window, you can often still punish their tech anyway. Forcing an opponent who just teched to block pins them down for your rushdown. Also you may not be able to hit them in time, but it is extremely hard to escape a throw just after teching.
Examples and setups
Miyako: Airthrow in the corner after bnb. If they tech neutral or back you can dash in and 2A or quickly throw. If they tech forward you can 2A them during the window where they cannot block. If they do not tech you can dash in for an otg String.
Aoko: Ground throw. Neutral or back tech can be defeated by dashing and landing a meaty 2C or just throwing them again. Forward tech can be defeated by merely waiting in place and 2Bing them when they land from their tech.
Chaos: Ground throw near the corner. If they air tech, you can jump back and j.C to force them to stay in the corner. Any ground tech can be punished by 2A. Not teching allows you to set up a meaty A snake pattern for rushdown. You can also just 5B them for not teching and land an easy invalid j.C loop.
Hisui: The infamous j.BB loop near the corner. This has several options for when the opponent does or does not tech and is an example of how tech punishing adds many options to your characters game that could not exist otherwise.
Kohaku: j.C near the corner in BNBs. Any tech option can be defeated with 2C. Not teching allows an easy OTG string. Teching the OTG string allows her 2C xx combo. Not teching the OTG string gives her a chance to set up meaty EX cactus.
Any character: The otg/double otg setup. Whenever your opponent does not tech and you go for an otg string, they have an opportunity to tech right after. This is punishable the same way you punish other techs. You can also end your first otg string early to see if they are holding for a tech in case you miss your full otg string, wait for them to pop back up, and punish. Once they dont hold a direction, you can start an otg string, end it early, dash back in, and do another otg string. And then when they expect the double otg, you can do the first part of your otg string, wait for them to pop back up, and punish again. This tech punish mindgame with otgs can be done by almost anyone.
Conclusion
Tech punishing allows you to prey on the opponent's fear for free damage, and at the high level you can use the fear of your tech punishes to set up long rushdown patterns that are actually techable, but the opponent chooses not to tech for fear of your tech punish damage. Master these techniques and it will add to your damage and mindgame.