It's his Last Arc. It's performed when you're in either Heat or Blood Heat and successfully shield, in Tohno Shiki's case, it's a miniature of his Arc Drive, which is quite nice for juggling.
MB Re Act is an older PC version of the series. There are a couple of newer versions. There is Melty Blood Act Cadenza for PS2, but it has only been released in Japan and you need either a modded PS2 or a Japanese one. And then there is Melty Blood Act Cadenza version B for PC, which is the newest one of them all.
The newest PC version is the best since it does not require any changes to your PC. You can buy either online on Ebay, or aa online Japanese import game site.
I wish I had someone to play this against -3- All my friends suck at this game. There´s only one whom I can take a longer gamer against. But he always plays nero. And it´s easy to counter him if you always meet the same person.