The rolloff like other said, but also it's absolutely critical that you source be "in front of you" contained within +/- 45 degrees so that you aren't picking up equal and opposite in the rear lobes, like you said.
If you run close to stage, flush with the PA, then you will pick up on stage stuff with the front lobes, and PA with the rear lobes, and yes, you will get cancellation. If you run in the middle of a reflective room, you will get PA in the front lobes and reflections in the rear lobes, and cancellation.
So it's a good tool, but it has to be used correctly. I haven't got the hang of it myself. I've probably tried it 6 times, and got 2 keepers and 4 throwaways.