Energy drink speculation:
Info gathered from page:
+ Bullets miss you
+ Some downside once it wears off that lasts for "several seconds".
Speculation:
+ Other projectiles might not miss you(fire, rockets?) but we can't really be sure
+ You might run slower afterwards(the downside)
Also look at the background, can anyone say "double payload"?
mirror (because the servers are being hit hard)

Also amazing how you can say what it is with complete certainty after just reading the extremely vague unlock page, Sirrus.