MB6582 CS: "Test LEDs now"

[emerson]

Following the MB6582 Control Surface Construction Guide, I'm nearly to the point of soldering the pins for the encoders.  The guide then offers the advice "Test LEDs now if you wish" and references J2 (the one near the power supply on the main PCB, I guess?) but I'm not sure what specific action is meant by that.  How would this notional LED test work?  Thanks in advance.

 

(Edit:  for posterity, I now realize it's talking specifically about the optional LEDs for illuminated knobs.)

Edited April 28, 2021 by emerson