The Napsack is a part of the Exploration Kit in Pikmin 2. Its function is to provide a quick method of returning to the Pikmin Onions or Research Pod, which is achieved by holding the X-button for a few seconds once it has added to your kit. The Captain you are in control of at that time will fall asleep and be carried back by the nearest Pikmin, up to four, a minimum of just one being required. However, one may ask, "Why not walk back there? It's useless!"; in actuality, the pilot should be carried back as fast as if all flowered White Pikmin were holding another item, regardless of color. The practical application of this is that it saves precious time: while one Captain is being returned to the rocket, the player can control the other, thereby getting a little extra done. Also, a Dweedevil can grab you and, if it falls off the edge in certain caves, a glitch will occur.
This ability is similar to that in Pikmin, which is activated by holding the D-pad. 4 to 5 Pikmin subsequently carry Olimar, in doing so rendering him impervious, though not under control - or so it is said - to attacks.