A pennant is a compressed symmetrical-triangle: highs fall, lows rise, and the shape resolves quickly. It differs from a bull-flag mainly in that a flag drifts in a channel while a pennant contracts to a point.
Because it is small and forms quickly, a pennant gives a very tight stop, which is the whole attraction. The flip side is that small patterns are noisy and produce more false triggers than larger ones.
Pennants are also where chart-clutter does real damage: on a fast chart you can find one nearly anywhere. Requiring a preceding impulse of a defined size, such as a multiple of atr, is what separates a pennant from ordinary chop.
Related: bull-flag, symmetrical-triangle, continuation-pattern, consolidation, false-breakout