The pole is the impulse; the flag is a tight pullback that slopes gently against it. Volume is typically heavy in the pole and light in the flag, which is read as profit-taking rather than aggressive selling.
The break above the flag is the trigger, the stop usually sits below the flag's low, and the conventional target is the pole's height projected from the breakout, a measured-move. The appeal is the geometry: a small stop next to a large potential target.
Two things void it. If the pullback retraces most of the pole, it is not a flag, it is a reversal in progress. And flags in the late stage of an extended move fail far more often than flags early in a trend, which no pattern description captures. Location matters more than shape.
Related: bear-flag, pennant, flag-pattern, measured-move, continuation-pattern