It increases the speed that AntiAir Shot shoots at, it's not really necessary and will probably annoy you in everything else you do :O! (chatting, etc)
I tried a new skill build, it's working FLAWLESSLY, but again because I'm a Hunter so I have some more skills that make it worthwhile, basically I re-skilled to the following (lvl 25):
Rising Arrow 4
Anti-air Shot 5
Bow Mastery 5
Advanced Bow Mastery 2
Acid Arrow 1
Arrow Shower 2
Bird Watching 2
It's a great skillbuild, I don't use Shootdown at all, and relaunch was useless for me so I canned it.
Basically I Arrow Shower a mob, Multishot to kill the first few and Rising Arrow->AAS the rest. If there are razor crabs I Acid Arrow & Bleed them. Although that's obviously after your a Hunter and past the scope of this tutorial.
Even with Arrow Shower however, this skill build works okay for a lvl 20 build.