Hi Jussi,
The relief carving showed an ancient blade which may or may not be the predecessor of the keris. Also, we do not know how the ancients used the earliest kerises, but staying in the here and now, we do see silat groups (i.e. Malay/Indon martial arts schools) which demonstrate the use of kerises as we know today. I'm in Singapore, somewhere in the heart of Southeast Asia, home to the keris, and I'm fortunate to see some of such silat demonstrations in person. Some of my Malay friends are actual silat practioners or are good enough to be in a position to instruct in silat.
I tried searching for a youtube video on how the Malay/Bugis keris is used, and I think the following is a good example to show how kerises are used now, and by extension, the likely position the hilt would have assumed for the purpose of fighting.
http://www.youtube.com/watch?v=sDFgG_LhKlg