I found this explanation of the Guard Beam at...
www.hoosiertire.com/dragman.htm
It states the following:
"Guard beam: A light beam-to-photocell connection located 16 inches past the stage beam that is used to prevent a competitor from gaining an unfair starting line advantage by blocking the stage beam with a low-installed object such as an oil pan or header collector pipe. If the guard beam is activated while the stage beam is still blocked, the red foul light is triggered on the Christmas Tree and the offender is automatically disqualified."
The best explanation is when Kurt matted the throttle, the torque of the engine twisted the car enough allowing something on the lower front of the car to set off the guard beam and caused the redlight. No he did not move but the front of his car was too low and the guard beam caught him.