I don't know how germane this is to the design we are discussing, or how anatomically true the explanation is, but I thought it should be mentioned. This is from a discussion of a cut out on a Philipine blade from tanaruz's thread
visayan kris http://www.vikingsword.com/vb/showthread.php?t=28209 post #2 in the Ethnographic forum ".....update on the Visayan kris. Since this was a 'mystery blade' to us(me and my father)-having an 'open groove' in the middle of the blade, we sought the original blacksmith in the hinterlands of Iloilo (and oh, it was scary because of some 'insurgents'). The kris' design was the blacksmith's signature design- to identify it being from his hometown. It was also designed, he said, to lighten the blade and most especially- if 'stabbed into the body (lungs, in particular) it would cause the collapse of the lungs and thus instant death.' "