This is done horizontally:

It can also be done:

How the pose can affect the power will be shown later...
The technique is a combination of Nero's & Dante's slash.
'Crimson Assault' can only be done unarmed. It involves a series of roundhouse kicks with a finishing uppercut kick to send the opponent into the air.

I had difficulty showing this one - this is a copy of Dante's moves when Gilgamesh is equipped (look for Dante vids on Gametrailers.com)