1. Use Vorpal Thrust while standing behind an enemy.
2. Use Impulse Drive while standing at on the side of an enemy.

If it isn't working, you're either missing one of them, or you're going too fast. Remember, there is positional lag in everything you do. Even though on your screen it appears you've moved to the side, the server still thinks you haven't finished moving yet. Move to the side, give the game a second to catch up, then use Impulse Drive. One sure fire way to make sure the game has gotten the memo re: your current positioning is to let an auto-attack go off before you try to use your WS, just to make sure the battle log says "Your attack hits so-and-so from the right (or left) for..."