From looked at DMC’s cutscenes and thought “they’re cool, but are they awesome and interactive?”. Obviously, no. Thus, Ninja Blade was created. Also, they decided to make QTE’s fun.
The story: mutant worms are taking over Tokyo. You are a Ninja; you have a Blade; your blood is poisonous to mutant worms. You must kill them all before the place is nuked from orbit. It’s pretty awesome.
Here’s how it works: every weapon can cause QTE attacks on ground enemies, and they’re completely optional. Y pops up above the enemy’s head, signaling the trigger. Should you choose to engage it, you get a 20% bonus on blood crystals (read: money). Should you get the first button press right, the bonus increases to 30%. Should you get the second button press right, it’s further increased to 50%. If you miss a button the enemy simply dies and you lose the opportunity to increase your bonus. If you manage to chain several of those you also get a life orb bonus.