The word of the day is Bump & Run Coverage.
Bump & Run coverage is a technique used by defensive backs in man to man coverage. They attack the WR at the snap of the ball which disrupts the timing and footwork of the receiver. After the initial contact they turn and run/mirror him in coverage.
Legal contact can only occur within five yards from the line of scrimmage. Disrupting the passing route hurts offenses that are dependent on timing routes because it delays the release into the route.
At times this type of coverage is called press coverage or tight coverage.