Read the Beginner's Guide. (FIGURE IT OUT DO BETTER!)
It's a two stage process. You land Baku Guard cancel and then choose a card. If it hits this seals movement, Specials, or puts them in Counter hit state.
If you land a card, you can do a followup that then seals a button of your choice if that move hits. There's a move for each choice.
This is useful if you see yourself returning to neutral after Baku or perhaps getting the final bit of damage in. If you are good you can get full combo off using Tsuki and then doing the right things.
It's tough, because even if you hit Baku there's no promise you'll land a card or hit certain followups.