Tenchu IV, the target shakes your hand, laughing heartily while exclaiming "Ohhhhh, you! You almost got me that time! I gotta watch this guy!" Of course we're kidding -- he tries to murder you. In situations when you've been noticed, rather than just giving up or retrying, you now have the option to actually fight in a new first-person mode.
In alternating attack and defense phases, you use Wiimote gestures to simulate a one-on-one swordfight, battling until one of you is dead or your sword breaks. It's like Rose & Camellia, except with to-the-death ninja fights instead of Victorian-era face-reddening! Being a ninja and therefore a total jerk, you can also throw knives while fighting.