Using bees/wasps to kill isn't the greatest tactic, but the Careers were not expecting it, and two people died from the stings. She used it as a gas bomb of sorts, but instead of gas, there are killer mutated wasps that cause insanity and illusions when stung. She carefully, so carefully, as to not wake or disturb the tracker jackers, plucked the nest out of the tree, and dropped it like a bomb below. The nest hit one branch on the way down, as Katniss was in a tree, causing her to get stung once or twice. But the nest served its purpose, causing havoc and scattering the Careers and killing two other tributes in the process.
But, for the people who don't like reading, the Tracker jackers were utilized as a bomb of sorts, dropping killer wasps onto the unsuspecting Careers.