[63corvette]

This is the defining of the starting line.
Maybe we are looking at this different for the same thing.
There are two beams with 2 senders and 2 receivers/reflectors for a total of 4 but not 4 beams crossing the individual lane. Only 2.
This was pasted from "Beards Reaction Time Clinic."

The Starting Line
The starting line is composed of two pair of photocells, one for each lane. The photocells detect your car when your tire interupts the light beam that crosses the lane. Tracks used to use bright lights and photocell receivers, but now many tracks are using the Compulink timing system which uses infrared beams. These beams are much better for drag racing. Drivers used to paint their wheels or put shoe polish on them to dull the bright, reflective surface. The old lights and photocell combination would sometimes pick up reflected light from the wheel, and then not "see" the tire. The infrared beams have no such problem. The timing system down the track uses a system of beams and reflectors, too. When the beam is complete (ie, the beam is reflected back to its origin), nothing is blocking it. When the cycle is broken, the beam has been interupted, and thus a tire has been sensed.

The two photocells are linked to the Tree: the Pre-Stage, and the Stage lights. When the car inches forward and blocks the first photocell, the Pre-Stage light comes on, indicating that the driver is close to the starting line. Most tracks have a rollout, or distance between the Pre-Stage and Stage beams of 6-8". The driver can now nudge the car forward until the car blocks the Stage beam, thus lighting the Stage bulbs on the Tree. This means that the driver is on the starting line, and is presumably ready to go.

I also stage as shallow as possible but I do not remember ever blinking the lights.
Some racers bump in using the transbrake and this causes the car to rock with the slack in the drive train which, in my opinion causes the blinking stage lights in most cases. I try not to let this bother me but it always seems to throw me off by not moving my stare to the starting lights. Just my problem I guess but it distracts me.
I am not sure how the stage lock system works. I do know it is part of the computer program and was an upgrade after the original program had been used for some time. I am not sure all tracks systems even have the upgrade or they choose not to use it. I just know at some races I see a car go red blinking the stage lights and other tracks it does not. I would just like to have the same game at all the races so I know what to expect.
By the way I am old enough and do remember when NHRA used a flagman to start the races.
My first races with NHRA was electronically timed but the starter noted and triggered the red light if you left to soon on the green and the finish line winner was judged by two officials. Each lane was timed individually and written on a card the size of a business card with a W or L indicating who won or lost. You received only your own time with a W or L. I still have a few of the cards and my first Class Win Trophy long before they were Wally's.
Just My 2 Cents
Rick Cates