I'm not sure how I did it, actually. In story mode I recruited Tezuka, and at some point he left. I don't know why, but it wasn't the normal "I'm pissed off so I'm leaving voice". I just left the slot open instead of recruiting someone else. Before the tournament he came back with SS in every stat, and when I got him to Tension level 5, he went Muga no Kyouchi. It gives you an infinite super bar :O
Looking at the FAQ, it's the "That man towards Kyuushu" event, but I have no idea how to trigger it.