I know that if we shoot narrow beam of electrons at the crystal (we don't change the angle) we will get a diffraction of electrons according to the brag's law. Therefore we get minimums and maximums on the screen around the crystal. Until now we have been dealing with problems where ##\vartheta## (this is an angle between an incoming beam and crystal planes) was constant. It meant that crystal wasn't rotating. To find maximums and minimums we have instead been rotating a sensor around the crystal. But what would happen if we rotated the crystal? This would mean that we are changing the angle ##\vartheta##! Would minimums and maximums move around the screen or would they only appear and disappear? What is different than before? A good picture is worth 1000 words.