Why would you tech backwards in a corner?
It's not about timing your attack so that you don't have to work for an easy tech punish. It is always going to be based off reaction of where they tech.
After airthrow, just wait. If they tech, you get punish, until they learn to stop teching. When they finally do, you get free Oki. It's not hard to react techs and punish with 5C, even that is more than enough for an easy tech punish.
If you got extremely good reactions, then just buffer 236 after you land, and press C if you see them tech.
Len is known for her tech punishes, that's why no one techs against her. It's also conditioning, and part of her mind games, but it's not free.