In a traditional seppuku you plunge the ritual dagger into your abdomen and cut from left to right. This is an incredibly painful procedure, and to finish the ceremony successfully you can't visibly admit to your pain. To accomplish this you have a trusted lieutenant decapitate you swiftly after the act is accomplished.

http://en.wikipedia.org/wiki/Seppuku will tell you more than you ever wanted to know about this ritual >.>