Quoted from Jesder:
You can only activate it during a battle with a monster (by hitting the target directly below the Black Knight).
It makes the shots in the battle worth more and the mode easier to complete. If you're going to use it in a battle, always best to make it the first shot of the monster battle as once you activate it, you will lose it once the mode completes or you drain.
Had a freakishly good game going today (two TKCs, three CMBs, and retro black knight). Was fighting the final monster before black castle, on ball three. That power Target was flashing it’s beautiful blue at me, so I figured sure, why not make this last monster quick and easy!? So, I nailed that target...boom, SDTM. Bye bye.