Note: This page is taken from n.wikia.com.
This feature of N is generally used ONLY when there is no way out of one's predicament, a situation known as undeath. The player can activate the suicide feature by pressing, by default, the "K" key in the game. This will cause the ninja to die - either by spontaneously exploding or apparently being sniped and crumpling to the ground/being thrown into the air (1.4 only) - and the player can then restart the level.
Of course, players who are not simply aiming to complete a given level and are going for highscores may also use the suicide function whenever they make a mistake in a run: oftentimes when highscores are involved, a single mistake marks the doom of any chances of obtaining a score. Thus, to avoid wasting time by completing the imperfect run and not obtaining the desired score, most players would opt to instead suicide and start the level anew, saving time.