In my opinion, “thank it forward,” is just another way of saying, “pay it forward.” For example, when someone does a good deed for me, I may choose to pay them back as a way of saying, “thank you,” to them. But instead, “Pay It Forward,” suggests that I should do good deeds for other people, which is kind of like saying that instead of paying back people for the good deeds they’ve done for me, I should do good deeds for other people as a way of saying, “thank you,” for people who’ve done good deeds for me. In other words, instead of “thanking them back” for good deeds that people have done for me, I’m “thanking it forward,” by “paying it forward,” and doing good deeds for other people.

Another way you can interpret, “thank it forward,” is: thanking people ahead of time; before someone does a good deed for you, as opposed to thanking them after the good deed is done for you. For instance, thanking someone in advance for doing me a favour, could be interpreted as “thank it forward,” as opposed to thanking them after they’ve done me a favour, in which case I’d be “thanking them back.” Although this a weaker, less meaningful interpretation that has no relation to “Pay It Forward,” it is, nonetheless, another way you can “thank it forward.”

I don’t see how that can work with “thank you”s.