A marksman shoots a Magnum .44 into a block of ballistic gel. In a slow-motion replay, you can see the aftereffects, in which the block has a case of gas, then suffers an attack of fiery indigestion, and then farts, through its newly-created "digestive canal." There's a perfectly normal explanation for this.
Slow motion video capture is a natural companion to just about anything that you’d need ballistic gel for, and good thing — because the video captured what appears to be a diesel effect! The block is hit with a bullet, and as the bullet rapidly expands and dumps its energy into the gel, a cavity expands rapidly. During this process, some of the (oil-based) material in the cavity has been vaporized. After the expanded bullet exits (to the right of the gif above but easier to see in the video below), the cavity in the block begins to collapse. The resulting pressure increase appears to ignite the vaporized material, which explodes with a flash followed by some exhaust.