If the laser is properly centered, only the center beam flickers as it encounters the data pits.
But if the disk drifts back, the front beam encounters the lane markers and starts to blink. And if the disk drifts forward, then the back laser hits the markers causing it to blink.
The blinking tells the motor driving the whole optical read/write head to move along the radius of the disk, re-centering the laser on the data track.
In addition, while the motor is catching up, there are also small side electromagnets around the lens holder that can momentarily push it towards the proper position.
