Mostly after you block moves which leave the opponent in a crouching state. The best way of learning to low throw punish is doing it on reaction in training mode.
Go for the easier to punish moves first, like Kasumi's 2H+K, or Jann Lee's 2H+K. They are quite unsafe on block, and very easy to throw. Like you said, Tina's 8P on NH (Normal Hit) knocks the opponent down so they can throw the opponent. Zack's 8P used to do the same in DOA2 and DOA3, but I don't know why they removed it (Actually I think you still can, I havent played DOA5LR in a WHILE).
You can also Hi-Counter Throw crouching OHs on reaction, and this applies for Tackle Throws (Mila's 66T, Tina's Spinning Throw, Leon's 33T, Bayman's 33T), which is very good to learn because those characters need those moves to function to their best ability. Even if you don't connect the throw from these attempted counters, the enemy's throw will not connect to you because:
1.) You are already crouching and the throw needs to hit a standing opponent
2.) Because of the Triangle System, the game will register your throw attempt as it had already beaten out the previous OH when it actually hasn't (From the last time I played it was like this)
You can crouching throw Brad Wong's 2P+K and any unsafe moves that lead up to it, and you can do the same for Helena's Bokuho stance (Though one of the most viable to punish is her 214P+K).
Also as you are a Tina player, your 3T is an OH, and you can try and react to counter any moves which have crouching state properties in them. It works, trust me.