**Pyroflatulence**

Well you did ask. And it is a real thing.

> Pyroflatulence or flatus ignition, is the practice of igniting the
> gases produced by human flatulence, often producing a flame of a blue
> hue, hence the act being known colloquially as a "blue angel", "blue
> dart", or in Australia, a "blue flame". The fact that flatus is
> flammable, and the actual combustion of it through this practice,
> gives rise to much humorous derivation. Other colors of flame such as
> orange and yellow are possible with the color dependent on the mixture
> of gases formed in the colon.
> https://en.wikipedia.org/wiki/Fart_lighting

Depending on the individual's gut bacteria, farts can contain large quantities of hydrogen and methane, both of which are highly flammable.

You can of course find many examples of this on YouTube. 

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**How can this be activated?**

We know that in times of fear or stress, people are much more likely to fart. This would make the weapon available at exactly the right moment. Just carry a pocket lighter. Perhaps there is some evolutionary adaptation that causes a spark to light the fart.

**Edit**

Inspired by @Agrajag to explain the spark generating method. I suggest that these people are also contortionists and can create the spark by gnashing their teeth together. They can be identified by their lack of eyebrows.