The term "butterfly knife" originated from the Filipino language as "Balisong," which means "broken horn." This knife has a split handle that--when closed--conceals the blade. The blade is flung open using several different techniques when it needs to be used. Over time, the butterfly knife handle will become loose, typically because of excessive use. When this happens the pivot pins on the handle need to be tightened back down. This process can be accomplished with ordinary tools.
Open the latch located on the bottom of the butterfly knife. Pull both handles out to the side, forming a horizontal line with the knife blade perpendicular to the handles.
Hold the handle and tighten the pivot screw using a suitable screwdriver or Allen key. The pivot screw is located where the handle meets the blade base. Note that since all butterfly knives vary, you'll have to choose which tool works to tighten the screw.
Repeat the process for the other handle. Fold the butterfly knife back together.
Open the knife back up and check to ensure that the handles aren't overtightened. The butterfly knife needs to be able to open quickly, and shouldn't get stuck in motion as you're trying to open it. Ensure that the knife still opens easily. If not, slightly loosen the pivot screws until the knife swings open efficiently.