Pichuun! is a rule that can be ignored at the DM's discretion.
Whenever a character is reduced to 0 HP, knocked out, killed, or otherwise becomes removed from a fight, they instead explode with a "Pichuun!". The character may exchange some brief banter with the one who defeated them, then disappears for an hour, catching up with the party later or whatever. They may recover hit points by expending hit dice at the end of this period.
This rule makes for a more relaxing and cinematic game. If this doesn't suit the game, the DM is free to ignore it.