Gentlemen, you might as well be asking why one group of hip-hop kids wears their ball caps backwards and another wear them to the side. As most of you probably know, Indonesia, though now one country, is made up of many different, even if related, cultures. Over hundreds of years and numerous religious influences different traditions arise in these various cultures. But if you want to go searching for a deeper reason why one island wears it's keris on it's back and another to one side or another you will most likely end up just hurting your brain. There probably was some reason hundreds of years ago, but i highly doubt you will find one now. As for the use of the keris as a weapon, there is clear evidence that when the Chinese first encountered the Javanese in the 14thC that the keris was more than just ceremonial there. But the keris was certainly never intended as a main weapon of war, but more as a person side arm not unlike the European rapier. The rapier is not a weapon one would take into serious battle. Neither was the keris. The Bugis were known to be more likely to actually use their keris as physical weapons and this can probably be seen in the stoutness and general simplicity of their blades. As you can see from the photos provided, the Balinese wore their long blades up their backs with their often bejeweled deity hilts facing foward as if they were sitting on the owner's shoulder (to whisper in the owners ear?). IMO i doubt this was for the purpose of any kind of quick draw as statis keris such as these were probably never drawn for fighting. Though i am sure i will recieve argument here, but the keris as a traditional fighting blade that has traditional (more than a hundred or so years) martial forms seems highly unlikely to me. I don't believe i have ever seen any old or ancient text on keris fighting (if you got 'em, smoke 'em boys ). It seems a RELATIVELY recent invention to me. This does NOT mean invalid ot irrelavent, but if we are trying to connect the reason for wearing positions with martial movement i think we are probably on the wrong track. Even though the keris does have a history of some actual martial use it's statis as a symbol of power and authority has always been stronger. The answers to what position the keris is worn in probably has a whole lot more to do with that then how quickly it can be drawn in battle.