The horizontal exhaust diverts the hot gases away from the helicopter and prevents the tailboom from heating up. This in turn helps reduce the helo's heat signature. There may be other advatages as well. However, those turned exhausts do "ugly" up a truly beautiful helicopter if you ask me. Better to be alive than pretty I guess though.
Ray
Here's the turned exhaust:
" border="0" />
Here's the old style exhaust:
" border="0" />