Operating position of halogen bulb?

[k00zk0]

I was wanting to build one of the "fire flashlights" that have been demonstrated before but was wondering about whether the fact that the operating position of the Osram 64623 being "s 90" could cause any problems. This seems to mean that you can turn the bulb 90 degrees in any axis from straight up; it can't work upside down. I don't see a reason why the bulb would be specified to not work upside down, except for maybe the action of the halogen gas inside the bulb being compromised causing it to burn out prematurely. Can someone shed some light as to why the bulb is not specified to operate upside down?

Thanks in advance.