If a team attempts a two point conversion after scoring a touchdown and the ball is fumbled and recovered by the defense or if a pass is intercepted by the defense who then run the ball back to the other end zone the defense team will be credited with the two points. This is called a defensive conversion.
Note that this rule only applies in College Football. In the NFL, the defense can't score on a point-after attempt.
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It is a scoring play used as a substitute for a point-after kick - the ball is advanced into the endzone like a touchdown play but it comes after the touchdown.