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.
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.
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.
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.
How this ability evolved
There was once a contortionist who worked at a circus. She was attempting a particularly difficult trick it involved a position that I'll let the reader imagine as it's too difficult to describe. Unfortunately she found an attack of wind coming on. Gritting her teeth she tried to avoid this but the result was worse than anything she could have imagined. The teeth grinding coincided with a tremendous f@rt.
The resulting burst of flame caused such a reaction from the audience that the ringmaster asked her to keep in her act. She did and the crowds flocked to see this amazing phenomenon. The contortionist eventually married a man who suffered from serious wind and between them they established a dynasty of f@rting contortionists. Initially this family specialised in circus but there came a time when one of the family developed a martial art based on the ability. From that point there was no stopping them.