People are probably using a play with an auto-motion, and the auto-motioned receiver runs a drag route. This play KILLS man to man D if the receiver has any amount of speed or acceleration.
If you see this play coming, switch to the corner covering the motioned receiver pre-snap, and if the WR gets set in motion, quickly run your CB to the inside of the field (the same direction the WR is going). Make sure that the CB is further to the middle of the field than the WR he is covering, so that when the ball is snapped, he already has a good jump on the route.