There were some of these similar tables that were converted to dual mode, but most users that converted, just went over to almost 100% CNC use.
The down side, of course to a tracing table is that it takes double the work space.
Simply, the principle of the trace is that the retro-reflected light back from the table is received by a photo-cell that oscillates, therefore if the reflected image back straddles the line, a sine wave with equal amplitude above and below the zero line is output from the Photo cell, If the head relative to the line moves off course, the sine wave zero reference moves up or down and a correction signal is sent to the servo to maintain the sine zero balance.
It is alot easier to explain with diagrams.
Normally a spot light allow the initial lining up of the head, and when the start button is pushed, the auto trace takes over, if a valid line is read.
The set up of the head is critical, initial hight etc.