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.
So like, would the same thing happen if I got shot? Because I don't want any bullets inside of me and I DEFINITELY don't want any explosions. Farting is risky enough I got fire insurance on my memory foam mattress topper.
Keep going for the whole video.
Thanks to RC, who only participates in spear battles because he's oldschool. Heck yeah, I only throw rocks.